Using a transistor to switch a circuit on... how?

Question

I have two sperate circuits I need to connect. Both have their own power source and I need to keep it that way. I have two leads that i need to switch power on where a SPST switch is located on the other circuit.

Simply, I have circuit A which needs to be turned on by circuit B, while keeping the power sources seperate.

Circuit B is a VOX switch that applies 4.5v to a remote lead. I need to take that 4.5v and use it to just complete a circuit on Circuit A without applying voltage.

Can I do this with a transistor? I have not been able to. Maybe I have the wrong transistor for this purpose?

I was thinking, use the 4.5v to switch the transistor which would complete the circuit for the switch on Circuit A. But it doesn't work when connected.

To make it very simple, two leads from one circuit need to complete a switch on another circuit without transferring the voltage... I'm just not thinking clearly of why it isnt working.

http://img114.imageshack.us/img114/677/s...

Thats the circuit and the circuit I need to make for it using a relay.

I have relay 275-232 from radioshack. Its a 5v reed relay. But I only have 4.5v switching voltage, which wont turn the relay on. The relay is good, it works with 5.5v applied. But I cant get the 5v to the leads, i only have 4.5v there.

Let me know if you need the values for the circuit components, i didnt put them on.

more slack · Accepted Answer

From the looks of it, you don't have the driver transistor connected properly. It isn't getting any power.

All you need is to use either a common-base or common-emitter to drive the relay. Also, don't forget to put a back-EMF diode on the coil, or it will damage the transistor.

Link below looks like a circuit for exactly what you want to do: close a relay based on voice input. At the very least take a look at how the transistor connects to the coil.

"more" stuff... why bother with the SPDT? If you put the relay in the same case as the other stuff then you have a pair of dry contacts to control the other piece.

Also, I found an even better VOX circuit (second link). It uses a single-ended supply op-amp and 4.5V (see pp. 18).

? · Answer

If an opto-isolator is already too large (4-DIP) and too costly ($1.20 per piece), then any other idea will be so, too. You can do this without opto-coupler or relay or pulse transformer, but then at least one potential of both power supplies need to be connected to each other, usually 0 Volt. If I knew more about your "trigger circuit" and what your "transistor relay circuit" has to perform, then I might be able to give a more precise answer.

billrussell42 · Answer

sorry, still having problems understanding this. I see what looks like a DPDT relay in the upper schematic (is that A or B?), but where is the coil for that relay? or is it a toggle switch?

Where is the 4.5 volts coming from? is that the "leads in" on the schematic?  Is the relay in the lower part (B?) part of the relay in the top part?



please label the schematic re your written description. Part A/B, the 4.5 volts, etc.

.

Using a transistor to switch a circuit on... how?

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