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.


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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

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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.

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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.


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