Time-delay relay uses proximity control

Dennis Eichenberg, Parma Heights, OH -- EDN, 3/15/2001

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 IC_1A, 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. V_CC 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.