Abstract: A scanner unit on a double-sided printed circuit board has reed switches on one face and coils on the other. The copper patterns include large screening pads beneath the reed switches and the coils. These pads distribute heat evenly along the switches to minimize thermally-induced E.M.F.'s, and provide electrostatic screening. An energizing circuit for the coils provides a single, long pulse several times larger in magnitude than the minimum required for relay actuation, followed by a series of shorter pulses of the same magnitude, but having a time-average magnitude equal to the relay holding current. This avoids the need for a voltage-dropping resistor and power wastage therein.

Abstract: A source for providing a voltage or current which is (remotely) controlled by a digital signal includes a digital-to-analogue converter (30) which supplies a proportion of a reference voltage (V.sub.R) in accordance with the digital signal. The gain of an amplifier (48, 50) receiving an oscillatory signal is controlled by an error integrator (32) in dependence upon the difference between the converter output and a feedback signal derived via a precision rectifier (62) from a secondary winding (60) on a transformer (56), the primary (54) of which is coupled to the amplifier output, so the feedback signal tracks the converter output. Another (isolated) secondary winding (66) and precision rectifier (70) provide an output voltage equal to the feedback signal; the output voltage can be used to control a voltage-controlled current source (74). Although the output voltage or current is d.c.