| Electronics > Projects, Designs, and Technical Stuff |
| Find highest value of 4 inputs without microcontroller? |
| (1/12) > >> |
| petersanch:
Hi All, I have 4 analog inputs and wish to find which one out of the 4 has the highest voltage. A TTL output is preferred. Feels as if there should be an IC that does this. Are there clever ways of doing this without a micro or A2D? Cheers! |
| mark03:
Six comparators followed by some digital logic? (One comparator for every signal pair: AB, AC, AD, BC, BD, CD. Then work out the boolean logic for your desired output, whether 1-of-4 or two-bit binary encoding.) |
| helius:
So with 4 inputs, there are 6 relevant comparisons: A vs B, C vs D, A vs C, B vs D, A vs D, B vs C. out(A) = (A>B)∧(A>D)∧(A>C) out(B) = (A<B)∧(B>D)∧(B>C) out(C) = (C>D)∧(B<C)∧(A<C) out(D) = (C<D)∧(B<D)∧(A<D) You can factor the comparisons into sums of terms, too. How about two LM339 quad comparators, with the outputs ran into a 74HCT132 quad 4-input NAND? |
| unitedatoms:
For positive only voltages, connect each input to anodes of leds of 4 optocouplers. Tie all led cathodes to single grounded resistor load. The ttl outputs of optos will provide 4 bit code. Where possible most expected values are 0001, 0010, 0100 and 1000. Other values must be decided upon too, use code into any combinatorial manner, for example 2 x 16 inputs demuxes with hardwired inputs if 2 bit output is required. |
| bson:
I second the optocoupler idea. The LEDs may differ ever so slightly in forward voltage, so that will be your main source of inaccuracy. Ra//Rb biases the LEDs, and if you want to bias at -Vf = -2.5V then Vss < GND - Vf < -2.5V. If you don't care about biasing just remove Ra and replace Vss with ground. |
| Navigation |
| Message Index |
| Next page |