# Need help: 741 op amp and speaker problem

#### jatchan

Joined Apr 4, 2008
6
I bought a Movement monitor few days ago. How the monitor works was that when it doesn't detect a movement in 20 seconds, the small build-in speaker beeps (only beeps when alarm is on) and the voltage though it was roughly 1VDC as I measured. In order to get more than 5 volts to flash my micro controller when alarm turns on (it needs >3V), I just simply built a non-inverting 741 op amp with voltage gain of 5 (Vout=Vin*(1+R2/R1), Supply Voltage + = +9V, Supply Voltage V- = -9V. I intercepted a wire from the speaker to the Vin+. Supposedly I should receive ~5V at the output of the op amp. However, I can only receive ~2V (keep changing) even if I increase the voltage gain. What's wrong with it? Can I just cut the speaker off from the board and connect directly to the Vin+?

#### Wendy

Joined Mar 24, 2008
21,914
Are you sure you're not looking at an AC signal? Check it with your voltmeter on the AC scale.

#### rwmoekoe

Joined Mar 1, 2007
172
i guess bill's right, it is ac, as you mentioned that it is a speaker, so the signal going through it should be the beeping ac signal.

just tap it out from before the decoupling capacitor, go through a common emitter, then a capacitor to hold it, kind of a peak detector, and an emitter follower. you got it.

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#### jatchan

Joined Apr 4, 2008
6
Thanks for your help! I measured it again by AC, the Voltage range was 0.9V to 1.3V. BTW, I don't even need a op amp to do it? When the alarm is ON, the digital output is always 5VDC? what about when the alarm is OFF?

#### rwmoekoe

Joined Mar 1, 2007
172
when the alarm is on, the output is 0v. when the alarm is off, the output is near 5v. it's easier to just use a transistor, because the supply voltage is limited, and you need to measure near 0 volt.
by the way, lessen the value of the input resistor if it misses the alarm.

#### jatchan

Joined Apr 4, 2008
6
Thanks again. However, I'm creating a alarm detector. Something like when the alarm is off (speaker not sound), the output will be near 0V. Once alarm is on (speaker beeps), the output is near 5VDC and stay until the alarm is off. Is there any way i can do it?

#### jatchan

Joined Apr 4, 2008
6
I am sorry for my unclear explanation.
Actually the monitor is powered by a 9V 100mA adaptor. I am not sure if the voltage drop across the speaker is AC or DC because the voltage keep changing from 0.9V to 1.5V on my voltmeter. Current is around 3.8mA.
The microcontroller is powered by a 12V 700mA adaptor. In order to send something out by the microcontroller, input with a logical HIGH (more than 7V) is need to perform the action. The microcontroller won't do anything if less than 7V going into the input.
I tried using a non-inverting opamp with voltage gain of 6, supply voltages +V=+9V, -V=-9V by two separate batteries. I tested the circuit using a 1.5V battery, it gave me around 8V. So I think it's ok. When the alarm of the monitor is on, I supposed to get 1.5V*6~8V at the output of opamp. Instead I just received 2.5V to 3V. How come...?

#### rwmoekoe

Joined Mar 1, 2007
172
there's nothing wrong with the maths, it's just that you are using the wrong measuring device. try an oscilloscope, or scopemeter. please post the results.

it is an ac signal. there's no need to use an opamp, because you'll have to peak detect it and feed it to a comparator too in the end. the circuit i posted earlier will do the job. just adjust the values a bit if it doesn't work.
try it. unless you want to walk the winding road.