This is a simple circuit which I built to one of my audio amplifier projects to control the speaker output relay. The purpose of this circuit is to control the relay which turns on the speaker output relay in the audio amplifier. The idea of the circuit is wait around 5 seconds ofter the power up until the spakers are switched to the amplfier output to avoid annoying “thump” sound from the speakers. Another feeature of this circuit is that is disconnects the speaker immdiatly when the power in the amplifier is cut off, so avoinding sometimes nasty sounds when you turn the equipments off.

Output Relay Delay Audio Amplifier Circuit Component list

C1 100 uF 40V electrolytic
C2 100 uF 40V electrolytic
D1 1N4007
D2 1N4148
Q1 BC547
R1 33 kohm 0.25W
R2 2.2 kohm 0.25W
RELAY 24V DC relay, coil resistance >300 ohm

Circuit operation

Then power is applied to the power input of the circuit, the positive phase of AC voltage charges C1. Then C2 starts to charge slowly through R1. When the voltage in C2 rises, the emitter output voltage of Q1 rises tigether with voltage on C2. When the output voltage of Q2 is high enough (typically around 16..20V) the relay goes to on state and the relay witches connect the speakers to the amplifier output. It takes typically around 5 seconds after power up until the relay starts to condict (at absolute time depends on the size of C2, relay voltage and circuit input voltage).

When the power is switched off, C1 will loose it’s energu quite quicly. Also C2 will be charged quite quicly through R2. In less than 0.5 seconds the speakers are disconnected from the amplifier output.

Notes on the circuit

This circuit is not the most accurate and elegant design, but it has worked nicely in my small homebuilt PA amplifier. This circuit can be also used in many other applications where a turn on delay of few seconds is needed. The delay time can be increased by using bigger C2 and decreased by using a smaller C2 value. Note that the delay is not very accurate because of simplicity of this circuit and large tolerance of typical electrolytic capacitors (can be -20%..+50% in some capcitors).

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The stoboscope tube needs about 250-400V DC to operate. This high voltage is generated using simple voltage step up circuit built from transistors Q1,Q2 and transformer T1. This circuit gives out about 230V AC voltage which is then rectified with rectifying bridge U1 (must have at least 400V voltage rating) and stored to the main capacitor C1.

I built my stroboscope from parts taken from old camera flash unit. This approach gave me nice flash tube with reflector, trigger transformer and some of the capacitors (for example C1). Other parts were the one luying around. I used some strange triac I found in my for the triggering circuit, but any triac which can handle at least 1A and 400V should do the job well.

12V Stroboscope 250-400V DC Circuit Component list

R1 1.2 kohm 1/2W
R2 1.2 kohm 1/2W
R3 120 ohm 2W
R4 10 kohm 1/2W
C1 100..150 uF 400V flash capacitor (from camera flash unit)
C2 4.7 uF 400V
C3 1 uF 400V
CZ 33 nF 400V
Q1 2N5983 NPN power transistors
Q2 2N5983 NPN power transistors
Q3 MAC216-4 Triac
TUBE Xenon flash tube (taken from camera flash unit)
T1 220V to 2×9V transformer, at least 10W power rating
T2 Xenon flash tube trigger transformer (from camera flash unit)
T3 Thyristor trigger transformer with good isolation
U1 Rectifier 1A 400V (can be build from four 1N4007 diodes)
S1 Switch one of three, at least 2A 400V rating

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Home security project
Securiy is a matter of great concern to all of us in this world. Normally there are four types of security threat to our property.

1 )Thief, an inturder.

2)Fire, so we use fire sensor.

3)Water spilage /over flow, so we use water sensor.

4) Gas leak , we use gas sensor, whenever there is security breach from one of above or any combination of above.

The project takes the following action

1) Sounds an ALARM to attract your home Security project
or public attention .

2) If the reset sw is not pressed within 30 seconds the projects takes further action,

3) It makes an emergency call to deliver an emergency message stored in speech IC.

4) It will make five attempts to call you before it checks the conditions again.

Atmel at89c51 Office-Home Security System

The project presented here is based on world’ s most powerful intel’ s mcs-51 family of microcontroller atmel at89c51. CIRCUIT EXPLANATION: This project is based on 8051 microcontroller.(IC2) IC3 and is used as buffer. IC4 is a one time programmable(otp) chip where messages are stored. This is 21 second speech ic where total of 12 messages can be recorded on eight different locations, but total duration should not be more than 21 seconds. Recorded messages can be played back by setting the trigger pin 10 & 11 to high, making these pin low will stop the message, Setting these pin high will repeatedly replay the same message. IC5 is an audio amplifire . Audio output from speech ic(IC4) pin no 7 cout is coupled to this ic on pin no 3 through VR2 volume control and C10. Amplified output from pin no 5 drives the speaker through capacitor C14. R14 and C13 corrects the tone. .C15 and C16 couples the audio message to telephone line. IC4 needs 3.3v operating voltage .R9 is a voltage dropping resistor, D7 is 3.3v zener diode and C7 is a filter capacitor. These components will always maintain the voltage to3.3v at pin 9,12 of IC4. capacitor C8 is a feed back capacitor.,C9 and R13 connected to pin no 7 of IC 4 are tone corrector.R12 along with the VR1 variable resistor performs the sampling rate adjustment. R10 and R11 connected to pin26 and pin 27 of the microcontroller are voltage dropping resistors. R15 and R16 connected to the base of transister Q1 and Q2 are voltage dropping resistors and drives transistors Q1 & Q2 when set to high logic by microcontroller. Q1 Drives the dial relay RL1and Q2 drives the off-hook relay.

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