Infra-red Light Barrier Using 555

This is a short-range
light barrier for use as an intruder alarm in doorposts, etc. The 555 in
the transmitter (Figure 1) oscillates at about 4.5 kHz, supplying
pulses with a duty cycle of about 13% to keep power consumption within
reason. Just about any infra-red LED (also called IRED)
may be used. Suggested, commonly available types are the LD271 and
SFH485. The exact pulse frequency is adjusted with preset P1. The LEDs
are pulsed at a peak current of about 100 mA, determined by the 47 Ω
series resistor. In the receiver (Figure 2), the maximum sensitivity of
photo-diode D2 should occur at the wavelength of the IR LEDs
used in the transmitter. You should be okay if you use an SFH205F,
BPW34 or BP104. Note that the photo-diode is connected reverse-biased!
So, if you measure about 0.45 V across this device, it is almost
certainly fitted the wrong way around.



The received pulses are first amplified by T1 and T2. Next comes a PLL (phase lock loop) built with the reverenced NE567 (or LM567). The PLL
chip pulls its output, pin 8, Low when it is locked onto the 4.5 kHz
‘tone’ received from the transmitter. When the (normally invisible)
light beam is interrupted (for example, by someone walking into the
room), the received signal disappears and IC1 will pull its output pin
High. This enables oscillator IC2 in the receiver, and an audible alarm
is produced. The two-transistor amplifier in the receiver is purposely
over-driven to some extent to ensure that the duty cycle of the output
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If the transmitter is too far away from the receiver, over-driving
will no longer be guaranteed, hence IC1 will not be enabled by an alarm
condition. If you want to get the most out of the circuit in respect of
distance covered, start by modifying the value of R2 until the amplifier
output signal again has a duty cycle of about 50%. The circuit is
simple to adjust. Switch on the receiver, the buzzer should sound. Then
switch on the transmitter. Point the transmitter LEDs
to the receiver input. Use a relatively small distance, say, 30 cm.
Adjust P1 on the transmitter until the buzzer is silenced. Switch the
receiver off and on again a few times to make sure it locks onto the
transmitter carrier under all circumstances. If necessary, re-adjust P1,
slowly increasing the distance between the transmitter and the

Light Gate With Counter Using 555 And 4033

The circuit described
here counts the number of times that an infrared beam is interrupted. It
could be used to count the number of people entering a room, for
instance, or how often a ball or another object passes through an
opening (handy for playing shuffleboard). The heart of the circuit
consists of – you guessed it – a light gate! Diode D1 is an IR diode
that normally illuminates IR transistor T1. The light falling on T1
causes it to conduct to a certain extent. The resulting voltage on the
collector of T1 should be just low enough to prevent the following
transistor (T2) from conducting. This voltage can be adjusted within
certain limits using P1.

As soon as an object comes between D1 and T1, the light shining on
T1 will be partially or fully blocked, causing the IR transistor to
conduct less current. As a result, the voltage on its collector will
increase, producing a brief rise in the voltage on the base of T2. This
will cause T2 to conduct and generate a negative edge at IC1. This
negative edge will trigger the monostable multivibrator, which will then
hold the output signal on pin 3 ‘high’ for a certain length of time (in
this case, one second). At this point, two things will occur. First, a
buzzer will be energized by the output of IC1 and produce a tone for
红包扫雷苹果下载地址 approximately one second.

When the buzzer stops, a negative edge will be applied to the clock
input of IC2, causing the counter in IC2 to be incremented by 1. IC2 is
conveniently equipped with an internal binary-to-BCD
decoder, so its outputs only have to be buffered by IC3 and T3 to allow
the state of the counter to be shown on the 7-segment display. Switch
S1 can be used to reset the counter to zero. If a one-second interval
does not suit your wishes, you can modify the values of R3 or C1 to
adjust the time. Increasing the value of R3 lengthens the interval, and
红包扫雷苹果下载地址 decreasing it naturally shortens the interval.

The same is true of C1. When building the circuit, make sure that T1
is well illuminated by the light from D1, while at the same time
ensuring that T1 ‘sees’ as little ambient light as possible. This can
best be done by fitting T1 in a small tube that is precisely aimed toward
D1. The longer the tube, the less ambient light will reach T1. The
sensitivity of the circuit can be adjusted using P1.

Voltage Controlled Switch using 555 Timer


In this circuit the 555 timer is used in a novel way, as a voltage
controlled switch.The old and omnipresent NE555 can be very good at
something it was not meant for: driving relays or other loads up to 200
mA. The picture shows an example circuit: if the input level rises over
2/3 of the supply voltage – it will turn on the relay, and the relay
will stay on until the level at the input drops below one third of the
supply voltage.

If the relay and D1 were connected between pin 3 and ground, the
relay would be activated when the input voltage drops below one third,
and deactivated when the input voltage goes over two thirds of the
supply voltage. It is also a nice advantage that the input requires only
about 1 uA, which is something bipolar transistors can’t compete with.
(This high impedance input must not be left open.) A large hysteresis
makes the circuit immune to noise. The output (pin 3) can only be either
high or low (voltage-wise), and it changes its state almost
instantenously, regardless of the input signal shape.


Circuit diagram

The voltage drop across the NE555’s output stage (at 35-100 mA) is
0.3-2.0 V, depending on the way the relay is connected and the exact
current it draws. D1 is absolutely vital to the safety of the integrated

555 Timer Clap Switch

This 555 timer clap
switch circuit electronic project is designed using some common
electronic parts. This 555 timer clap switch circuit electronic project
operates from a distance of up to 10 meters from the microphone . Signal
from microphone is amplified by transistors T1, T2 and T3. Diode D1
detects clap signals and the resulting positive voltage is applied to
the base of the transistor T4 .


Circuit diagram

T5 transistor will amplify the output signal from the T4 transistor
and will trigger the monostable multivibrator based on the 555 timer IC.
The output signal from the 555 timer IC is used as a clock for a 4017
decade counter. For each successive clap T6 conducts and cuts off
alternately , resulting and on of switching for the lamp . Triac used
for this project can be a 8T44A or ST044 and can drive load up to 4 amp
rating . The 12 volt for powering this electronic circuit project is
derived directly from the mains using rectifier diode D2 and some other
components like R16 and ZD1 zener diode.

Alarm Circuit for Snoring Prevention by 555 Timer

The circuit was designed
to produce a circuit that will alarm a sleeping person to prevent
snoring by using a vibrator instead of an audio alert so as not to
affect the whole household. It contains a trigger indicator, peak
display indicator, a level control and a variable trigger threshold. A
small motor enclosed in a film case with of 35 mm in size, will provide
the vibration and suitably positioned under the pillow or mattress.

  • Snore – the outcome sound of an obstruction in air passage during
    breathing while sleeping which causes the respiratory structures to
  • Light Emitting Diode (LED) – a semiconductor diode that is commonly a source of light when electric current pass through it
  • 555 Timer – an 8-pin electronic device used in several mixtures of
    applications involving multivibration and timing operating modes
  • Operational Amplifier (Op-Amp) – a differential amplifier having a
    large voltage gain, very high input impedance and low output impedance
  • Electret Microphone – a type of condenser of capacitor microphone
    that utilizes a permanently charged object to eliminate the use of a
    power supply
    The operation of the alarm is depends on several adjustments of the
    components. The variable resistor VR2 will designate the preset period
    of triggering the alarm while variable resistor VR1 controls the volume
    of the snore. The threshold control will set the triggering of the alarm
    since a snore is a continuous sound lasting for several seconds.

It has a set delay so it will not activate with short noises such as
car horns, doors slamming, and others. As the circuit gets activated,
the vibrations will work gently to wake the snorer or force him to
change his sleep posture. To illustrate the scenario in general, the
circuit may be divided into four partitions, according to the sequence
of operation, using a low pass filter, precision rectifier, delay-on
circuit, and timer and motor drive.

An electrets condenser microphone functions as the input transducer
to the amplifier and low pass filter around the op-amp circuit of IC1 to
filter out high frequency noises by reducing the amplitude of
frequencies higher than the frequency response limit of the system. It
will only allow the passage of low frequencies.


Circuit diagram

The precision rectifier made by op-amp IC2 converts the amplified
sound to DC which will be filtered again. This should go on for a few
seconds so the delay circuit will be activated. Op-amp IC3 comprises the
delay circuit and will function as a level shifter by comparing the
reference input set by the threshold control VR2 to the charge on
capacitor C8. The timer and motor drive will be triggered upon reaching
the threshold. The potentiometer R15 can be made to adjust the delay of
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To illustrate the operation of the circuit in detail, it starts as the sound is received by the ECM
microphone followed by amplification from the op-amp IC1, reducing high
frequency gain and acting as an active low pass filter. It is possible
to utilize a dynamic microphone with the elimination of resistor R1. The
gain at low frequencies is inversely proportional as frequencies rise
above 1 kHz and the level is controlled by VR1.

The conversion of audio signal happens in op-amp IC2 as it functions
as a precision rectifier to boost the signal levels by having a gain
ratio of R7/R6. The feedback loop contains the diode 1N4148 that is
accountable for producing a positive rectified signal from the
conversion of audio signal. The non-inverting inputs of op-amps IC1 and
IC2 is biased by C2, R4 and R5 to half the supply voltage. The visual
indication of peak levels will be supplied by LED1 by showing a flash
instead of continuous illumination. These peak signals are fed by R8 and
C5 to the LED1. The flashing of LED1 by each snore is modified by VR1.

The delay is crucial to the circuit so that the alarm will not be
triggered with any form of background noise. It will only be triggered
after the snoring is started. The need for an input delay is important
so the alarm will not set off in the middle of the night with a car door
opening or a car horn. The alarm employs high frequency roll-off so as
not to get affected by other sound with fundamental frequencies or

C8 and R12 provide the input delay. Capacitor C8 has a value of 33
uF as an electrolyte capacitor. It will start to charge slowly when
using the half wave rectified signal from IC2. Without any signal, C8
will not charge and will discharge via R11 and R12. The combination of
R9, R10 and D2 provides further rectification of the input signal and
causing 1N4148 diode D2 to conduct with a little forward bias. This will
also cause C8 to pre-charge even without a signal.

Since op-amp IC3 functions as a variable level detector, it also
provides the delay while the threshold is controlled by VR2 for the
capacitor C8 to have a voltage charge equal to the pin 3 of the op-amp.
With this event, LED2 will indicate the triggering of the circuit which
will cause the normally high IC3 to change to low output. The charging
of the capacitor can be computed only when a fixed DC current is used
but will not be possible on this circuit since the intermittent snore
provides the charging current.

The delay circuit output is normally high on during the triggering
stage and will change shortly when the prolonged snore is being
identified. The change will trigger IC4 555 timer because of the correct
polarity as the IC functions in monostable mode. A delay of 24.2
seconds can be obtained from the values of C9 and R15. Loads of up to
200 mA can be driven by the 555 timer output while transistors Q1 and Q2
can source up to 3 A. The power dissipated on the load and will not
require heatsinks when both transistors are ON.

During the construction of the circuit some key points should be
considered. A motor with high power and high torque should not be used.
Similarly, the motor must not exceed 1 A of current from the power
supply. However, a 9 V or 12 V electric motor is preferred. A resistor
can be added in series with the motor if it is producing excessive
vibration. Using a multimeter while the motor is running can measure the
value of the DC current.

If the motor would draw a current less than 200 mA, as supplied by
the 555 timer, then it won’t be necessity for R16, Q1 and Q2. Short
flashes are produced by LED1 to indicate the peak detection of the
sound. This detection can be adjusted by VR1 which will charge capacitor
C8 slowly. The threshold to allow the circuit to trigger after a few
seconds is adjusted by VR2. This triggering is indicated by LED2.

The capacitor C8 will start to decompose during the interval between
snores. Because of this, the circuit will not give false alarm with any
surge of short noise. Snoring can be caused by a lot of factors and
reasons the can be out of our control such as allergies, asthma, a cold,
sinus infections, being male, being middle aged or beyond, or
hereditary. It can also be within our control such as sleeping posture,
alcohol or medications, a history of smoking, and being out of shape or

The blockage in irregular flow of air may be due to obstruction in
the nasal passageway, fat gathering in and around the throat,
mispositioned jaw due to tension in the muscle, and throat weakness
红包扫雷苹果下载地址 which causes the throat to close during sleep.

Despite of this snore alarm circuit, there are still natural ways of
preventing or reducing the snoring like losing weight, clearing the
nasal passages, avoiding certain foods, medications and alcohol before
bed, elevating the head of the bed, and sleeping on your side. A circuit
similar to this can also detect if an infant sleeps on his back.

555 Touch Switch Circuit

This circuit uses a 555
timer as the bases of the touch switch. You can learn more about 555
timers in the Learning section on my site. When the plate is touched the
555 timer is triggered and the output on pin 3 goes high turning on the
LED and the buzzer for a certain period of time. The time that the LED
and the buzzer is on is based on the values of the capacitor and
resistor connected to pin 6 & 7. The 10M resistor on pin 2 causes
the the circuit to be very sensitive to the touch.

555 Timer Circuit With Variable On/Off Times

This circuit enables the
on/off times of a 555 timer to be independently varied over a wide
range. This is not possible with a conventional 555 circuit with the RC
network being charged from the positive supply rail and discharged via
pin 7. Instead, the capacitor at pins 2 & 6 of IC1 is charged and
discharged from the output at pin 3. Furthermore, the charging and
discharging circuits are different, being isolated by diodes D1 &

Circuit diagram:

555 Timer Circuit With Variable On/Off Times

555 Timer Circuit Diagram With Variable On/Off Times

Therefore the capacitor at pins 2 & 6 is charged via diode D2
and trimpot VR2 and discharged via D1 and trimpot VR1. With this
arrangement you can have very long on times combined with very short off
times and vice versa, or you can adjust the duty cycle to exactly 50%
and so on. This circuit also employs a second 555 timer (IC2) as an
inverter so that complementary pulses are available, if required. If
not, delete IC2.
Author: A. Davies – Copyright: Silicon Chip Electronics

555 DC/DC Converter

It is all too often
necessary to augment the power supply of an existing electronic circuit
because exactly the voltage that you need is missing. The circuit
presented here may provide a solution in a number of cases, since it can
be used to convert a single-ended supply voltage into a balanced set of
supply voltages. That’s not so remarkable by itself, but the special
feature of this circuit is that this is accomplished without using
difficult to obtain, exotic ICs. All of the components used in the
circuit are ones that every electronics hobbyist is likely to have in a
drawer somewhere.

The heart of the circuit is formed by an ‘old reliable’ 555 timer,
which is wired here as a free-running oscillator with a frequency of
approximately 160 kHz. The oscillator is followed by two
voltage-doubling rectifiers, consisting of C1, D1, D2, C3 and C7, D3,
D4, C5. They are followed in turn by two voltage regulators to stabilise
the positive and negative voltages generated in this manner. The duty
cycle of the 555 is set to approximately 50 percent using R1 and R2. The
square-wave signal at the output of the timer IC has a DC offset, which
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555 DC/DC Converter

555 DC/DC Converter

The amplitude of the output signal from the 555 is approximately
equal to the supply voltage less 1.5 V, so with a 12-V input voltage,
there will be a square-wave signal on pin 3 with an amplitude of
approximately 10.5 Vpp. With respect to ground (across R3), this is this
+5 V / –5 V. Although this yields a symmetric voltage, its positive and
negative amplitudes are somewhat too small and it is not stabilised. In
order to split the square-wave signal into sufficiently large positive
and negative amplitudes, C1/D2 are added for the positive voltage,
causing the positive half to be doubled in amplitude.

For the negative half, the same effect is achieved using C7/D3.
Following this, the two signals are smoothed by D1/C3 and D4/C5,
respectively. Both voltages are now high enough to be input to normal
5-V voltage regulators, yielding symmetric +5V and –5V supply voltages
at the output. The input voltage does not have to be regulated, although
it must lie between +11 V and +18 V. The maximum output current is ±50
mA with an input voltage of 12 V. This circuit is an excellent choice
for generating auxiliary voltages, such as supply voltages for low-power
opamps. Naturally, the fact that the converter can be powered from the
in-vehicle voltage of a car is a rather attractive feature.
Author: L. de Hoo – Copyright: Elektor Electronics

555 Police siren

The Police Siren circuit
uses two 555’s to produce an up-down wailing sound. The first 555 is
wired as a low-frequency oscillator to control the VOLTAGE CONTROL红包扫雷苹果下载地址 pin 5 of the second 555. The voltage shift on pin 5 causes the frequency of the second oscillator to rise and fall.

Automatic Hand Dryer Circuit

now and again ardent hobbyists looking to add something special and individual to their rest room/bathroom want an automatic hand-dryer. this design guide exhibits that you need barely anything more than a handful of inexpensive components and the right enclosure. here is how to build a versatile hand-warmer at a low cost!

before you start the construction, make sure you know what are the various components and parts reside inside the elegant enclosure of an automatic hand-dryer (h-d). in principle, an automatic h-d consists of the following components and parts.

  • Power Supply
  • Main Circuit Board (Controller Board)
  • Hand-dryer Heater (Heater/Fan Assembly)
  • Hand-Proximity Sensor
  • Indicators and Switches
Automatic hand dryer internal view

internal view of an automatic hand-dryer

红包扫雷苹果下载地址almost all bricks of an automatic h-d can be home-brewed, but a dedicated h-d heater (heater/fan assembly) should be purchased from the external world. do a little more homework (google!) before taking your final purchase decision in this regard.

Hand dryer heater

schematic circuit diagram of our 230v ac operated automatic hand-dryer is shown here. since the electronic circuitry demands a stable 12v dc supply, make necessary arrangement to provide it from any self-made/ready-made linear or switch-moded power supply unit. the circuit is built around two lm555n chips (ic1&ic2);

Automatic hand dryer circuit

ic1 configured as medium-power inverting current driver, and ic2 as monostable multi-vibrator. when infrared light (ir) from the infrared light emitting diode (ir led) falls on the phototransistor (t1), by reflection, output of ic1 goes to high level and this will switch ic2 through the bc237 transistor (t2). as a result, the electro-magnetic relay – emr- (rl1) at the output of ic2 is energized to run the hand-dryer heater unit for a prefixed time period determined by the in-circuit values of rc timing components r5 and c5. the red led (led1) is the standby mode indicator, and the green led (led2) is the active mode indicator. the hand-proximity switching level can be adjusted with the 1m potentiometer (p1). the intensity of the infrared light source depends on the value of current-limiting resistor r2 (tested with 470r).

红包扫雷苹果下载地址both the infrared light source and the phototransistor can be mounted side-by-side on a single panel. but, the infrared light source (ir led) must be isolated from the phototransistor (t1) (using a small opaque sheet) to avoid false detection due to infrared leakage. the component values are not especially critical. however, the coil of relay rl1 must be have a low operating current, no more than a few dozen milliamperes.

Hand proximity sensor

红包扫雷苹果下载地址hand-proximity sensor’s location in the enclosure

the entire circuit should be fitted in a well-insulated enclosure, since it is usually installed in a watery-area (rest room, bathroom, etc).

courtesy note: some raw-images from internet, source including but not limited to