Sending tone through baby monitor

[LiteBright]

I have 2 baby monitor transmitters and 1 receiver. One transmitter will actually be used normally to transmit to the receiver to hear the baby. I would like to remove the microphone from the second transmitter and replace it with an IC (or some type of simple circuit) which generates a tone. This way I can send a tone to the receiver (to let the listener know that I need them) without disturbing the baby.

Does anyone know of an IC (or simple circuit) that generates a tone which I can replace the baby monitor's microphone with? The baby monitor's transformer puts out 12V AC, but I can use batteries if necessary.

[I_Daniel]

Do a google search for a 555 IC oscillator. They work from 5 to 15 volts and you can use a press switch to switch the supply on - similar to a doorbell switch - i.e press on , release off.

You will however have to use a volume control or preset a variable resistor in the output from the IC as the output from a microphone could be anything from a few millivolts to 1 volt. Start at the minimum output and slowly increase it until the tone is heard on the receiver. Without this attenuator you could blow the input of the transmitter.

You should also use a capacitor - 1 to 2 Mfd before the attenuator (volume control pot).

[LiteBright]

This circuit looks interesting. I assume the pot will replace the 330 ohm resistor but I'm not sure pot to try and where the capacitor should go:
[url]http://hop.concord.org/s1/core/s1ca1.html[/url]

[I_Daniel]

The circuit looks fine.
The capacitor could be upto 10 Mfd. connect it to the output of the 555. that is pin 3 and then you connect the pot to the cap and the other side to the negative line. Try a 1K Ohm pot.

Remember that the output from the 555 using 12 volt will be way too high. Some electronic VOM's can read anything from DC to a couple of MHz so it may be possible to first check the AC output and set it to about 50 millivolt. If you have an oscilloscope it is much easier to check and set the output etc. (The capacitor type microphones usually have a 1 Volt output.)