# Electret microphone is giving unsual resistance

#### rahulb

Mar 14, 2018
45
Hi,

I am trying to make a clap switch, schematic is given below:

I am using 9v battery but it is giving 7.55v currently. When I am measuring the voltage in microphone voltage divider part.
it is coming 6.7v. which is strange because If i calculate the resistance for the voltage then the microphone resistance is around 22K and it is without any sound. what I am doing wrong?

thanks

#### Audioguru

Sep 24, 2016
3,650
I calculated 26k ohms for the mic. Most electret draw double the current of yours resulting in a resistance measurement of about 10k ohms. Its resistance changes a tiny amount with sounds.
The 3.3k resistor R1 should be between 7.5k to 10k.

The very old 741 opamp was designed to use only a 30V supply but some work with a supply as low as 10V if it is biased at exactly half the supply voltage. Your supply voltage for the lousy old 741 opamp is too low and its bias voltage is much too close to positive supply voltage.

Also your CD4027 is connected wrong.

#### rahulb

Mar 14, 2018
45
I calculated 26k ohms for the mic. Most electret draw double the current of yours resulting in a resistance measurement of about 10k ohms. Its resistance changes a tiny amount with sounds.
The 3.3k resistor R1 should be between 7.5k to 10k.

The very old 741 opamp was designed to use only a 30V supply but some work with a supply as low as 10V if it is biased at exactly half the supply voltage. Your supply voltage for the lousy old 741 opamp is too low and its bias voltage is much too close to positive supply voltage.

Also your CD4027 is connected wrong.

hi,

thanks for the information. I want this circuit to be controlled by 9v battery. what opamp do you recommend for this voltage?

thanks

Sep 24, 2016
3,650
A TLE2141CP opamp works fine when its supply has dropped down to 4V and sells today for $1.92US at Digikey for the 8DIP package. #### rahulb Mar 14, 2018 45 A TLE2141CP opamp works fine when its supply has dropped down to 4V and sells today for$1.92US at Digikey for the 8DIP package.

thanks audioguru. I tried to get this IC but unfortunately it is unavailable in my area. Can I use any ic like LM358 or something else?

#### Audioguru

Sep 24, 2016
3,650
An LM358 dual opamp might work if you disable its second opamp.

#### rahulb

Mar 14, 2018
45
An LM358 dual opamp might work if you disable its second opamp.

disable means not adding any connection to second opamp?

#### Audioguru

Sep 24, 2016
3,650
To disable means to prevent an unused opamp from oscillating which causes an interference input to the other opamp. Disable it by connecting its (+) input to 0V and connecting its (-) input to its output. Then its output will always be close to 0V.

#### rahulb

Mar 14, 2018
45
thanks a lot audioguru. I will create a new circuit and test it.

but, you also told that 4027 ic connections are wrong. I copied as it is from a book. what you found wrong in it?

#### Audioguru

Sep 24, 2016
3,650
The CD4027 is a dual flip-flop. Deactivation of the second flip-flop by grounding its clock input and connecting the other inputs like the first flip-flop is not shown, to prevent interference.

A longer duration of the pulse from the opamp is needed to prevent an echo or other sound from instantly resetting the circuit.

#### rahulb

Mar 14, 2018
45
The CD4027 is a dual flip-flop. Deactivation of the second flip-flop by grounding its clock input and connecting the other inputs like the first flip-flop is not shown, to prevent interference.

A longer duration of the pulse from the opamp is needed to prevent an echo or other sound from instantly resetting the circuit.

if I deactivate second flip flop, short duration of pulse will work?

#### Audioguru

Sep 24, 2016
3,650
You must deactivate the unused flip-flop so that it does not cause interference to the main flip-flop and so it does not become damaged by static electricity.

If you let the pulse from a clap have a short duration then an echo or other sound will reset the switch when you do not want it to reset yet. You must have a simple circuit stretch the duration of each pulse.

#### rahulb

Mar 14, 2018
45
You must deactivate the unused flip-flop so that it does not cause interference to the main flip-flop and so it does not become damaged by static electricity.

If you let the pulse from a clap have a short duration then an echo or other sound will reset the switch when you do not want it to reset yet. You must have a simple circuit stretch the duration of each pulse.

do you mean, I should add a timer 555?

#### Audioguru

Sep 24, 2016
3,650
Try it first without stretching the duration of pulses. If you do not use it in a location that has echos and noise then it might work fine.
My daughter bought a cheap Chinese clap switch then asked me if I could stop it from quickly turning off the thing it turns on. It detected the echo of a clap. I did not bother stretching the duration its pulses.

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