Servo - December 2015

Figure 3 shows one way to configure two single pole double throw (SPDT) relays to control a motor (both relay modules in Figure 2 use SPDT relays). This configuration is called an H-bridge, and allows the motor to go forward when one relay is triggered, and reverse when the other is triggered. The two diodes in red allow stopping by either coasting or braking.

Coasting switches the motor off to slow down gradually. Dynamic braking stops the motor quickly by switching the motor leads together (very handy to avoid overshoot and/or to stop on a dime).

You can see the difference between coasting and braking at www.youtube. com/watch?v=n9PWwmQJfIs. You could easily make two relay H-bridges from a four-relay module, and your PICAXE’s four output pins could control the robot chassis shown back in Figure 1.

Transistors and MOSFETs are modern solid-state alternatives to electromechanical relays. They have the advantage of offering motor speed control since the PICAXE can rapidly switch them on and off using Pulse Width Modulation, or PWM. There is usually some voltage drop through a transistor or MOSFET, compared to little or no voltage drop through a relay. I won’t delve into transistor/MOSFET selection here, other than to say that TIP31s and IRF510s are pretty good parts to start experimenting with when you are so inclined.

A single relay or transistor is fine for spinning a motor in one direction, but when you need to reverse it, things get more complicated. The right side of Figure 4 shows a four-transistor H-bridge, required for forward and reverse. H-bridges also come as modules now which simplifies things. Notice that there are two inputs to an H-bridge: make one input high (and the other low) for forward, and vice-versa for reverse. However, it’s game over if you make both inputs high simultaneously. You get a pair of short circuits from V+ to ground which generate smoke and destroy driver transistors. Wiki “H-bridge” to read about “shoot-through,” and avoid it at all costs.

You’ll get your chance later building and programming RoboCar.

There are several affordable H-bridge modules to pick from. Figure 5 shows an 800 mA L9110S (left) and a 2A L298N (right). Both cost under $5 and have two separate H-bridges, suitable for driving two motors. The L298N offers dynamic braking as shown in the previously mentioned video.