Welcome to Manins' truck camper project

TX3100 UHF Radio

A GME TX3100 80-channel 5 watt UHF Radio now resides in the bin where once the diff. lock switches and lamps were (see here). Purchased off eBay, it came with a quality spring-mounted 6.6 dBi antenna.



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A lug on the bull bar accepts the bolt for the antenna. The coax cable, sheathed in 7 mm dia. split conduit, passes into the cabin through the large rubber cable entry gland above the starter battery and then over to the vacant upper bin in the dashboard.


New power connections, both ignition-switched and always-on, are also fed to the vacant upper bin. There the diminutive TX3100 is fitted to a bracket and rear connections, a remote speaker above the driver, and the microphone are plugged in.

The manual for radio operation is available from GME and here for reference.

Anytime ON for UHF Radio


If the radio is powered from the Always-On feed there is the possibility of inadvertently leaving it on, flattening the starter battery if on long enough. But powering from the Ignition-Switched feed means that the radio is off as soon as the ignition is turned off. The solution is a simple circuit that keeps the radio powered whenever the ignition is on and for a set period once the ignition is off. Its possible to also power up the radio for a set period at any time.

The circuit fits in a small box with a push-button on the front. Briefly pressing the button provides power to the radio for almost 10 minutes. Pressing the button again at any time extends the power for a further 10 minutes.

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It uses a CMOS NAND device with Schmitt-trigger inputs to monitor the voltage across an R-C pair with time constant of about 10 minutes. When that voltage decays to a low value the device output, which was low, goes high and switches off the p-Channel MOSFET to cut off power to the UHF radio.


I use just one of the four NAND devices in a CD4093B quad 2-input NAND gate with Schmitt Trigger. The CD4093B does not need a special power supply: it is powered directly from the starter battery. The MOSFET is a IRF9540N. It is capable of handling huge current, but here it only needs to switch a maximum of less than 1 Amp (the LOAD). It and all other components come from Jaycar.