On Apr 23, 5:51 pm, Bill M wrote:
Rob wrote:
dspurg7310 wrote:
I am really new at electronics projects. I am trying to build a repeater
out of two mobile VHF radios and I'm not sure how to solve a problem I
am having:

My main problem is with the radio that I want to transmit. It does not
have VOX mode on it and I'm not sure how to get the first radio to
trigger the second radio to transmit. Is there a simple way to do this
or do I actually need a radio with VOX capability?

You need to build a circuit that connects the Squelch signal from your
receiving radio to the PTT input of the transmitting radio.
This could be as simple as a transistor and a resistor, it could add
some timer(s) and tone generator(s), or it could be a complicated circuit
with a microcontroller.
It all depends on how you want the repeater to behave.

Ah, brings back memories! Permit me a few words of bandwidth.

I built our club's repeater in the 70s. WR4ARO, iirc. Don't recall
much but it was a Motorola Sensicon (A?) receiver strip and an RCA
transmitter strip. It wasn't quite as simple as tagging off the squelch
to key the xmtr, although that could work functionally. I built a COR
(Carrier Operated Relay) circuit from a 73 Magazine article.

The gist of it is that you need some 'hang-time' with a repeater to deal
with mobile drop outs, and you need to be able to sub in the callsign ID
at intervals and deal with timers and such to allow the thing to shut
off transmitting in the case of noise or a deadman on the mike.

Much easier these days since rptr circuits are more commonplace and
well-designed.

Think outside of the VOX box.

GL,
Bill

Hey OM:
VOX is easy. Take the 8 ohm speaker output, with a step up line
transformer, match the speaker output to the mike input.
Take a 1N914 rectify the audio from the mike input, unfiltered, to the
high capacitance input of a power FET. Put a 50K pot across the gate
to ground so you can adjust the "hang time". Use the FET to drive the
PTT.
Squelch the the receive radio, so now you gots VOX to the
transmitter.
Soon as the receive radio unsquelches it keys the transmit.

The caveat here is: if you interfere with a co-ordinated repeater, you
going to have to work it out, you might end up on the losing end of
that, because you can't become co-ordinated by a State co-ordinating
counsel.

73 OM
de n8zu