# Strange voltage variation in circuit

#### rahulb

Joined Jun 5, 2019
23
Hi,

I made a circuit which turns on according to sounds. actually it toggles. One loud sound, it turns on and in the next, it turns off.

When I measure voltage across the electret. it is around 6 v. I am using 9 volt battery for power which is producing around 8.5 volts.

Now the problem is, in case of a loud sound, the voltage is increased to 6.3 to 6.4 v and it remains there. then, on next sound in comes back to 6 v. The voltage doesn't change until it receives next sound.

I am not getting it what is going wrong? I am measuring voltage across the electret.

thanks

the circuit is below:

#### ci139

Joined Jul 11, 2016
1,383
Last edited:

#### Dodgydave

Joined Jun 22, 2012
9,093
I would put a 100nF cap in series with the output of the mic.

#### ci139

Joined Jul 11, 2016
1,383
I would put a 100nF cap in series with the output of the mic.
? and cancel your DC biasing for the opamp (if it were bjt input one it might somehow even work but you got the j-Fet input one)
slow down - there is no train to catch here

assume your mic. has impedance near-above 2kΩ and std. terminal votage near-above 2V e.g it's bias current is expected near 1mA
using 8.5V supply you calculate (8.5 - 2)·1V/1mA=6.5kΩ of which the mic. has 2.2kΩ thus you need biasing resistor of 6.5 - 2.2 = 4.3kΩ
then your inverting input has a DC bias apx. of 2.2kΩ / (6.5kΩ) · 8.5V ≈ 2.88V

since that bias depends on battery SOC and your specific electret response you may want to dc decouple it

but then you'll need to adjust the biasing for the opamp ... to smth. like (electronics.stackexchange.com/questions/...)

. . . but this is not all since the Vo will be an ~AC signal centered to near your supply median e.g. 8.5/2= 4.25V you need to make the AC amplitude to form a CLK pulse . . .
e.g. you need to rectify and discriminate it (basically pass through integrator of some sort) you might want to use a staircase schematic for this e.g ...

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#### Audioguru

Joined Dec 20, 2007
11,249
Your 10k resistor R1 that powers the electret mic is connected directly to the fluctuating +9V from the battery. R1 should have a series 1k resistor in series with it and the junction of the two resistors should have a 47uf filter capacitor to ground and the other end of the 1k resistor can be connected to +9V.

The lousy old 741 opamp was designed 52 years ago to use only a 30V supply. Many of them do not work with your 9V supply that drops to 6V during the life of the battery.

If the opamp works, since it has no negative feedback then DC and low frequency voltage gain is 200000 times or more which amplifies the slightest change of battery voltage. Also its output is clocking the flipflop thousands of times for each sound.

#### TeeKay6

Joined Apr 20, 2019
572
Hi,

I made a circuit which turns on according to sounds. actually it toggles. One loud sound, it turns on and in the next, it turns off.

When I measure voltage across the electret. it is around 6 v. I am using 9 volt battery for power which is producing around 8.5 volts.

Now the problem is, in case of a loud sound, the voltage is increased to 6.3 to 6.4 v and it remains there. then, on next sound in comes back to 6 v. The voltage doesn't change until it receives next sound.

I am not getting it what is going wrong? I am measuring voltage across the electret.

thanks

the circuit is below:

View attachment 183037
A 9V battery is not an ideal power source. If you draw current from it, its voltage will drop slightly. As the battery ages/discharges the voltage drop effect gets worse. In your circuit it is possible that one loud sound causes the relay to be closed (assume: coil is energized). The increased current to drive the relay coil causes the 9V (and all voltages derived from it) to drop slightly. When another loud sound causes the relay to open (assume: coil is not energized), the drop in current allows the 9V to rise again.

For any power supply, but especially with small batteries, it is important to bypass (put in parallel with) the battery with a capacitor. For your type circuit, two caps would generally be used: (1) a small capacitance, low ESR, capacitor (e.g. 0.1uF) and (2) a large capacitance, moderate ESR, capacitor (e.g. 100uF). The caps help to prevent rapid changes in the battery voltage and that is important for devices like the LM741 opamp and the electret mic. Even better would be to stabilize the battery voltage by passing it through a regulating device (a "voltage regulator")...but you would have to stabilize at a much lower voltage, e.g. 5V, if you want to use the full life of the battery.

#### ScottWang

Joined Aug 23, 2012
6,885

#### MisterBill2

Joined Jan 23, 2018
5,807
I see that only one section of the 4027 dual FF is used. If the other section is not being used then the inputs must not be left open, because that will lead to assorted undesired operation. That might be causing your voltage variation indirectly. On all CMOS devices all inputs must be connected and not left open.