Mindboards

Okay, cool.

It uses MOSFETs. They are not variable; they use PWM from the motor port to control the "power level". They are either all the way on or all the way off.

Well, when they are all the way on they act as a relay, without an external power source. And when controlling a motor, they are, in effect, acting as a variable resistance.

h-g-t wrote:...And when controlling a motor, they are, in effect, acting as a variable resistance.

There are PWM, not variable resistance. Either would indeed effect the speed of a motor, but PWM is better in many ways.

BTW, I have noticed that the Relay Driver outputs are either off 1 and 2, on 1 off 2, or on 1 and 2. There seems to be no way to only have 2 on. Have you noticed this?

I did say 'in effect'.

Behaviour in port A is different from the other ports and I have only tried port C, works fine there.

h-g-t wrote:...Behaviour in port A is different from the other ports and I have only tried port C, works fine there.

The H-Bridge of port A is different than the one used for ports B and C, and uses opposite polarity. That causes issues with the gate signals of the MOSFETs, as they are inverted.

I'm sorry if that was offensive. I meant it to be informative.

No apology necessary - I was just in a bad mood that night!

Although the driver can operate two motors at variable speeds but only in one direction, I think it should be possible to operate a single motor in both directions using a latching double pole relay.

This site explains the principles involved -
http://www.distel.co.uk/DC_MOT_CON1.htm

I have attached a possible layout for consideration. The relay could be driven from the same power supply as the motor using a voltage regulator.

Some latching relays need a set and reset pulses on separate pairs of wires. Others use only 2 wires but the pulses have to be in different directions. The type needed here is a 2- wire uni-directional pulse model. Also called a sequencing relay. Whatever you call them they tend to be expensive!