Switch for electrical circuit

Hello,

I'm helping a friend by taking this device which lets out a sound and making it light up at the same time as the sound. I thought this would be easy; however, I found that the voltage where I want to hook up to is normally high at +5V and only drops down to +4.7V when the device "beeps". I thought I would use this change in voltage as the input of an optocoupler and then build the circuit for the light on the other side. I'm having troubles with it because everytime I have worked with an optocoupler it has been with a device which went from +5V to 0V or vise versa. Anyone have any suggestions as to how I can make this work?

thanks!

You could amplify the small change in output from the photocoupler's transistor, then use the amplifier output to trigger the light.

That drop in voltage, if you did not look at it with a scope, could be an actual waveform. 2000-3000 Hz is common. A missing-pulse detector might work for you.

http://www.ecelab.com/circuit-miss-pulse-det.htm