Many people with significant physical disabilities can't operate everyday mechanisms, such as TV remote controls. To make matters worse, adaptive technologies are often unaffordable unless insurance covers them. This Design Idea describes an interface circuit that lets a disabled person control eight remote-control functions. The design uses older, small-scale-integration ICs because of their simplicity, low power requirements, affordability, and availability at stores such as Radio Shack. Because the circuit uses no microcontroller, you need not do any programming.
Power for the circuit in Figure 1 comes from four 1.5V AA batteries in series. Diodes D1 through D4 reduce the battery power from 6V to approximately 3.4V, and they protect against accidental reverse polarity of the batteries. IC1, a 555 timer, and associated discrete components form a repetitive-pulse generator. Potentiometer R1 adjusts the pulse speed. This pulse feeds directly into decade counter, IC2, which causes indicator LEDs LED1 through LED4 to sequence on and off. Each output of the decade counter feeds one input of CMOS gate IC3 and AND gate IC4. Normally, the output of the NAND gate is low because both inputs must be logic one to produce a logic-one output to close one of the CMOS switches, IC5 and IC6.
If the user presses the control switch while the desired LED is lit, both inputs to one of IC3's AND gates are at logic one, causing the output to be logic one and closing a 4066 switch, which is effectively the same as pressing one of the buttons on the remote control. As long as the control switch remains closed, the 555 pulses remain disabled and LED1 through LED4 remain in their current state. This characteristic is important because a person can continue to hold the control switch closed to continuously increment the changing of a channel or increase or decrease the volume.