The Microcontroller Idea Book - Jan Axelson's Lakeview Research

Chapter 11
Figure 11-1. Solid-state relays provide an easy way to switch power to AC or
DC loads..
Using a solid-state relay saves you the trouble of building a similar circuit from discrete
components. Surplus relays are inexpensive, as low as $1.50 each, from vendors such as All
Electronics, Marlin Jones, and Hosfelt. If you don’t have a data sheet for your relay, look
for a pair of control pins, usually labeled + and -. The other two pins connect to the load.
In Figure 11-1, the control bits are outputs of a 74LS374 latch addressed at E400h, as
described in Chapter 6. On a normally open relay, the load switches on when its control bit
is low. If you want a logic high to turn on the load, wire an inverter between the ’374’s output
and the relay, or use a normally closed relay. Or you can use a 74HCT374 latch in place of
186 The Microcontroller Idea Book