Time-delay relay uses proximity control
Dennis Eichenberg, Parma Heights, OH -- EDN, March 15, 2001
You can build a circuit that allows a passerby to briefly operate model trains in a display window (Figure 1). The design uses a proximity detector rather than a pushbutton switch to eliminate the need to mount and wire any equipment outdoors. The circuit worked well in this application and other applications. The heart of the circuit is the quad CMOS NAND gate, IC1. A human hand near the sensor induces 60-Hz power-line noise into IC1A, and this IC triggers IC2. IC2 is configured as a monostable multivibrator, with a period equal to 1·1(R3+R4)(C3). The period is adjustable from approximately 0.5 to 50 sec. The sensitivity of the circuit depends on the size of the sensor. A 10-in.-sq piece of screen mounted inside the display window works well for this application, because it permits complete visibility through the sensor. The circuit triggers several inches away from the window in this application.
You can manually or automatically operate the circuit through the single-pole, double-throw, center-off switch, S1. In automatic operation, receptacle J1 is active when IC2 drives relay K1 on. Lamp LP1 indicates when the load is active. Lamp LP2 indicates when the circuit is in automatic mode. J1 is active whenever S1 is in the on position, as LP1 indicates. VCC for the circuit comes from transformer T1, rectifier D1, and filter capacitors C1 and C2. The load rating for the circuit depends on the selection of fuse F1, switch S1, relay K1, and receptacle J1.
