Hi to all! New to this group, I pose a question: has anyone built or know of the best and easiest way to make a digital equivalent of a rotary switch? In a hobby application it would be great to have a keypad or bcd thumbwheel/s to select one of a number of output circuits. The limitation on rotary positions being about 32, it would be nice to have a circuit that was expandable to include a maximum of 100 positions. Output loads are 5v, 5-10ma, and could have transistors to increase current if needed. arjay
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Hmm... As so often there are more ways to skin a cat.
Thumbwheels with BCD-encoded output are common. With two thumbwheels you can make 100 different codes. The problem is the decoding. One way is to use
74HCT42 1 out of 10 decoders. To select 1 out of 100 you will need eleven of them plus some logic. The latter to make the enable/disable function which is not available in the decoders. Quite a lot of board space and soldering. An alternative is to program the whole decoder in a PLD. One with enough I/O pins. Too expensive for a hobby project I guess.
If you want less mecanics and more electronics you can build a BCD up/down counter to make the BCD code. One switch up/neutral/down and a slow clock will do the control but you'll need a display unit to make the selection visible. Two BCD-to-seven-segment decoders and seven segment displays. The decoding problem will remain the same.
To reduce chipcount for the decoding you can go binary and use 74HCT154 decoders. These are 24 pins beasts and you still need eight of them although one can be replaced by HCT42. Of course the counter needs to be binary as well. But... you will need digital-to-BCD conversion for the display unless you go for hex. The digital-to-BCD decoders are not available but you can easily program one in an EPROM.
One you start programming I'd go for a micro. All the control and counting can be done in software. The hardware will be 28-pins micro, two seven segment displays and seven 74HCT154.
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