Driving a relay from a radio

High tier MOTOTRBO radios can be used to control electrical equipment, either via the telemetry feature or via one of the other auxiliary (GPIO) outputs available on the back - or side - of the radio. More often than not, this is done via a relay.

The trouble is that, in most cases, one cannot connect the relay directly to the radio because all (electromechanical) relays require a fair bit of power to energise the coil and when the output is turned off, the collapsing magnetic field produces a back-EMF that will damage the radio output.

To drive such a relay, some interface electronics is required. This interface needs to convert the low level output from the radio into a higher current and voltage to drive the coil. In addition to this, the interface also needs to provide isolation between the radio and whatever is connected to the relay since, in many cases, whatever is on the other end might be mains powered.

There are a number suppliers for this hardware but a quick internet search brought me to this product which appear to use opto-couplers in addition to the driver transistors. This one has eight relays but there are other board with a different number of relays. Since this board uses transistors, you'll need to provide power.

Another option would be to use a Solid-State Relay (SSR). In this case, back-EMF is not a problem but most SSRs I've seen require around 15mA on the input which is right on the limit of what the radio can drive. Here too, I would suggest using an interface board like the one above.

The DM4400e has seven GPIO pins. GPIO1 can only be configured as an input and GPIO10 can only be configured for a limited number of output conditions. More information about what functions are available on these GPIO pins can be found on my Wiki.

GPIO10 can be used to drive a relay as it has the required (beefier) transistor and back-EMF protection. It however can only support functions related to horn and lights and PA relay.

When configured as an output, GPIO2-8 all have an internal NPN transistor (Rohm EMH25 HA) with a 4,7k resistor pulled to 5V. The transistor has a  maximum power dissipation of 75mW though I'd prefer not testing this limit.

Portable radios do not have GPIO like the mobile does but they do support a small number of telemetry inputs and output. To access these, a cable like the PMKN4040A is required. This has the identification chip which will tells the radio that an appropriate cable is connected. More information about this can be found on my Wiki.