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Old 12-25-2012, 9:00 PM
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Default Alternative to Relay in Repeater circuit

I'm building a mobile cross-band repeater using a 2M mobile station and a 70cm HT.

Getting the audio to repeat from 2M to 70cm was simple enough as I fed the audio output of the 2M radio to the external MIC input of the HT and turned on VOX.

Getting the audio to repeat in the other direction has proven more tricky. I built a simple VOX circuit that triggers a relay DPDT relay to simulate the PTT press and connects between the HT headset output and the MIC input of 2M radio. The issue I run into is that since the audio is switched by the relay, when the transmission ends, and the VOX releases the relay, the EMI generated by the relay switching triggers the VOX again which causes a feedback loop with the relay clicking on and off continuously.

So one side of the relay is switching the input audio, and the other is triggering the transmitter (by attaching a pin to ground).

I tried filtering the relay "click" with a capacitor, but it has been hit and miss.

My alternatives, are:
- Find a solid-state alternative to the relay that will pass the audio un-altered and provide ground
- Get rid of the VOX altogether and trigger the relay on carrier detection. (requires modification of HT)

Suggestions?

I know someone will suggest getting a dual-band mobile unit with cross-band repeat function, but I've come so far, I'd like to make this work.

ThanX
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Old 12-27-2012, 9:59 AM
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Have you tried putting a suppressor diode across the relay coil? A capacitor doesn't usually work well for snubbing the back EMF created when the relay is de-energized.
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Old 12-27-2012, 10:09 AM
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Thank you for the suggestion. I will try that.

I was really hoping to find a solid state alternative to the relay. Wondering if I could get away with using a FET.
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Old 12-28-2012, 8:27 AM
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Either a FET or a bipolar transistor will work, as long as they have the current capability. I've used 2N2222 or 2N3906 bipolars to key things. (Use the transistor that's controlling the relay to control the HT's PTT line directly. The HT probably takes less current than the relay.) Find out what's causing the retriggering, though - you can't fix it without knowing what it is that you're trying to fix. A storage scope will be the easiest way to see what's happening where.
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Old 12-28-2012, 11:49 AM
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As a point of clarification, I'm using the relay to trigger the PTT on the mobile set, as the mobile doesn't have a VOX function.

Based on the testing I've done, I'm pretty sure my problem is that relay is switching the audio that feeds the VOX circuit. I wish I could post a schematic of my circuit. It would probably help. I'm using the following VOX circuit:

http://www.kitsrus.com/pdf/k126.pdf

I've replaced the microphone with the audio output from the HT headphone jack. The DPDT relay (I replaced the AZ-SH-112L shown in the diagram) is then used to mimic the PPT switch on the microphone. In my mobile, one side of the relay triggers the transmitter (by grounding a circuit) while the other side connects the HT audio to the mobile's microphone input while disconnecting the actual microphone.

When audio is detected coming from the HT, it triggers the VOX circuit which simulates the PTT and sends the audio through to the mobile. This part works great, however when the audio stops, and the relay switches back, the noise from the relay switching feeds back to the input of the of the VOX circuit re-triggering the circuit.

Thank you for the help!
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Old 12-28-2012, 1:11 PM
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Try the diode across the relay. In fact, for future reference, you ALWAYS use a diode across a relay coil to eliminate transients caused when the coil is energized or de-energized. There is a back EMF created that can be in the hundreds of volts, that will take out the relay driver transistor, and put noise into everything else.
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Old 12-28-2012, 3:38 PM
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I went back to the circuit to add a diode, but realized that there is a 1N4004 diode across the coil of the relay. I think I'm going to put a FET in parallel with the relay and use the FET for the audio, and use the relay to ground the Transmitter/activate the transmitter.

I don't think the issue is the coil so much as it is the physical switching, and possibly the arcing when disengaging the transmitter. Might be better off with two SPDT relays, now that I think of it.
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Old 12-28-2012, 6:36 PM
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Just replace the relay with a FET to switch the mobile PTT line. Why pop the mic in and out when the rig is being used as a repeater?
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Old 12-28-2012, 6:42 PM
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I guess I was thinking I could retain the option of transmitting directly from the mobile transceiver even when the repeater is connected.
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Old 12-28-2012, 8:47 PM
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Been there done that, and pulled more than a couple of hairs. You have to figure out what is causing the VOX to trip when receive quits. A spike back from the relay coil is a likely suspect. You may also have a slight thump as the audio PA relaxes which will re-trigger the VOX. You should trace the source down first and attack it that way, but if all else fails you could use a capacitor to add a little debounce to your PTT driver input. If you can delay the PTT by 10 or 20 milliseconds it may be enough to ignore the clams that spike your input.

Last edited by ramal121; 12-28-2012 at 8:56 PM..
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Old 12-28-2012, 11:49 PM
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Have a look at this to see if you can improve the circuit.

Using Transistors As Switches
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Old 12-29-2012, 12:17 PM
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The audio "thump" is a real consideration in VOX circuits, so if that is the culprit, it will need to be resolved first. Anyway, if you still need a relay to accomplish the task, take a look at this little jewel. I use them on repeater-to-microwave interfaces. Reliable, cheap, and easy to integrate.

ELK-924 Sensitive Relay

Last edited by CCHLLM; 12-29-2012 at 12:22 PM..
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Old 12-29-2012, 5:56 PM
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VOX is not the best way to go with the cross band control especially if the frequencies involve repeaters with squelch tails. You would be better served to get a schematic of the radios and find the squelch transistor. Connecting a wire to this, buffer with a high impedance op amp, and you should be okay. If the radios are set for PL's, the keying will go stop and start with the PL.

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