The control is intended to switch mains-powered equipment in synchronism with a master control unit. One particularly useful application of it is in audio racks where the signal sources (cassette deck, CD player, tape recorder, tuner, and so on) are switched on and off together with the power amplifier.
The circuit monitors the current drain of the master unit (via K2) with the aid of optoisolator IC1. When P1 is set to maximum sensitivity (corresponding to the highest resistance value). a few milliamperes are sufficient for the control to switch on the slave(s) connected to K3. The maximum sensitivity should be used only rarely since allowance must be made for leakage and quiescent currents of the master unit.
When the current drain of the master unit exceeds the preset trigger level, the transistor in 1C1 starts to conduct, causing the output of IC2 to go low. Consequently, T1 .conducts and actuates the load (the slave) via relay Rel. After the master unit has been switched off, C2 is charged via R5. At a certain voltage on C2. the comparator toggles and switches off the slave via Ti and Re. This happens after 500 ms or so.
The state of the control is indicated by two ..EDs: one. D5, is the on/off indicator for the control proper and the other, D6, shows the on/off state of the slave unit. The maximum loads at the master and slave outputs are 500-W and 750 W respectively.
The control is best constructed on the PCB shown in Fig. 2. The mains connections to the board are made with three 3-way PCB terminal blocks. For safety reasons, the earth track on the board should be strengthened by a piece of copper wire with a cross-sectional area of 2.5 mm2 or larger.
WARNING. Since the board carries danger-ous voltages at a number of points, it is essential that proper electrical insulation is applied. Never work on the board when the mains is connected to it. Make sure that no part can be touched when it is being adjusted or used
Parts list
Resistors:
R1 = 120 Ω
R2, R5, R11 = 10 kΩ
R3, R10 = 1 kΩ
R4=33 Ω
R6= 68 Ω
R7 = 270 kΩ
R8, R9 = 47 kΩ
R12 = 2.2 kΩ
R13 = 220 Ω. 1 W
Capacitors:
C1 = 470 μF, 25 V
C2 = 47 μF, 25 V
C3 = 150 nF, 630 V
C4 = 10 μF, 25 V
Semiconductors:
D1-D4 = 1N5408
D5 = LED, red
D6 = LED, green
D7 = 1N4148
B1 = 840C1500
T1 = BC327
