BCD rotary switch: Binary-coded-decimal (thumb wheel) are not only relatively expensive but is also often not available to the wanted specification. A good alternative is shown in the present circuit that uses generally easily available components and a 12-position rotary switch (the most popular type) of which two positions are disabled. The terminals of the switch are connected to the inputs of a priority encoder. IC1. When an input goes low, the IC puts the number of that _ input as an inverted BCD code at the output. The four XOR gates enable the inverted code to be inverted again into a standard BCD code.
This operation is effected with link JP1: in position P, the output co carries the standard BCD code, and in position N, the inverted BCD code.
The circuit is powered by 3-18 V (because CMOS ICs are used). The use of a 5 V source enables LS. TTL, HC and HCT inputs also to be connected to the switch. The circuit draws a current of only about 200 μA.