Circuit not working...

So I made this circuit ( but it doesn't seem to be working. When I connect it to power the relay is in on position and the led is off. No matter how many times i clap it doesn't seem to change state. Can anyone please tell me what i could've done wrong? Note: when i touch the bottom of the circuit ( on some pins between 1 and 4 of ic 741) it seems to change state. Help would be greatly appreciated thanks. (This video: shows this circuit in action. It seems that he used an LED instead of a relay. I also found similar circuits here:

sort by: active | newest | oldest
shahryar.adil.3 (author) 10 months ago

So...after painstakingly checking each connection and looking for shorts, the circuit working. Turns out I had not connected the mic to pin 2 of the 741. This was by far the WORST functional project I have ever made. It refuses to turn on by claps (unless you clap a 100 times) and it only works when you blow air into the mic or hit it with something. There is no delay so it keeps on getting triggered multiple times. I searched alot but this was the only clap switch I could find which used a relay and didnt use arduino or a programmed pic chip etc. Could any of you please suggest a circuit that you think would work nicely by claps and not have any software involved? Thanks.

1. It would seem that your 741 isn't doing the job. You should investigate why you may have more faults.

2. Loads of clap switches, I haven't built nay though

3. Logical testing stage by stage is a huge part of building anything. A more important skill than being able to assemble a cct. In general a good pattern is to test for a known signal in the middle of the cct, if you don't find it go back towards the input and try again until you find the fault.

shahryar.adil.3 (author) 10 months ago

I found these three, do you guys think that these will perform better? (Ill use a microphone) (with only one relay connected)

rickharris10 months ago

1. we can't see what you have done, I guess, assuming the original project worked, you have a wiring problem.

2. Show a diagram of how you have connected the relay.

What look like it should happen is clap and the relay is on, clap and the relay is off and the LED is on - repeat.

Make sure you have connected exactly as the diagram. It looks like it should work.

use a multi meter to monitor the voltage at the transistor base is it changing.

Has the 4017 got power and ground.

have you linked 4 and 15 to reset the counter.

have you taken pin 13 to ground.

have you used exactly the same components.

shahryar.adil.3 (author)  rickharris10 months ago

Ill see if i have a wiring problem. The relay is connected correctly as it turns on when i connect the battery. the problem is that the input from mic does not do anything. Yes the 4017 has power and ground. Yes pin 4 and 15 are connected. Pin 13, Pin 8 and Pin 14 are all connected together and a wire is connected to pin 14 which connects with ground. Yes, I have used exactly the same components.The voltage at the transistor base does change when the relay turns on so ...

PIN 14 is not to ground it goes to the output of the OP-AMP !!

See -max-'s comments on how bad this clap switch was designed.

There is NO transient separation from the relay coil on the positive rail.

The 741 is possibly the worst, oldest and outdated Op-Amp to use.

There is No time delay preventing multiple re-triggering of the 017 IC from one clap oscillation.

The 017 is over-kill for a flip-flop.

Click the pic to see the Entire image..


So it looks like your problem is at the pre amp (the 747) end of things check with a meter that the output changes when the mic has an input.

The UA741 is a pretty old and crappy op amp, you should find tutorials on how to use op amps, and learn about what limitations they have, and which ones may be affecting your circuit and wiring. Below is a small list of what you need to be mindful of when working with the UA741

Input voltage range: About 2 volts within your supply rail. (Vee+2, Vcc-2)

Output voltage range: Ditto

Output current sinking / sourcing: 20mA

Offset voltage: 1mV, but externally trimmable

slew rate: 0.5 V / uS


input Z: 0.3Mohms

Inputs and output voltage constraint is the most important and easily missed. Make sure the input voltages well within this restriction. In general you should be mindful of how much current the op amp can source or sink. The datasheet specifies 25 to 40mA short circuit but for a valid (accurate) output I would avoid drawing more than 20mA. The offset can be a problem in some cases, but I doubt it is in this case. Slew rate and GBP express how 'fast' the op amp is, the UA741 is very slow but usable. Input impedance is also supprizingly low, it should be infinity, so be mindful that driving the input with a high impedance source could lead to significant errors.

-max-10 months ago
"Note: when i touch the bottom of the circuit ( on some pins between 1 and 4 of ic 741)"

Look at the datasheet, you will see that pins 2 and 3 are the inputs for your operational amplifier. The output will represent the difference of the voltages at these inputs, multiplied by a VERY large (but frequency-dependant) gain.

In the simplest model, you can imagine the output representing whether the non-inverting input is higher voltage than the inverting input. A comparator. Which is exactly how your supplied circuit is operating.


I believe that the problem is that you are exceeding the common mode voltage ratings of the input of your op amp. In other words, the voltages on the input are simply "out of bounds" and not between 3 volts above ground and 3 volts below Vcc, and you have chosen the resistors for the input incorrectly. If you can trigger the circuit by touching the input pins of the 741, then you may have selected the input resistors too high, the UA741 does not have very high Z inputs, so you should be mindful of that. Like I said in the last comment, the UA741 is a poor choice for beginners due to it's limitations, and if you are using it as a comparator, you should be using a proper comparator.


Consider using the LM339 or it's smaller cousin the LM393.