Please help me design a logic circuit that counts 35 interruptions of an IR sensor then opens a relay?

Question

Hello. Thank you for stopping by.

I need to design and build a little board that will close a relay via pushbutton (while ignoring an interrupted IR sensor), then count 35 interruptions of said sensor, opening the relay upon seeing the 35th count.

For perspective on what I'm doing here: this system is going to count the number of rotations of a gear then power down the DC motor driving the mechanism upon seeing the desired number of rotations.

Answer 1

While there are many ways to do this here's an approach I have used.  Take two decade counter IC's with a downward counting mode and preset inputs such as the 4029 and wire them to count in the down mode and wire them so they are cascaded together. This means that the overflow (actually underflow in the down mode) from the ones counter decrements the tens counter. Look at a data sheet for this part for specific connections. Then wire the preset inputs with the binary equivalent of decimal number 34, (not a mistake as 0 will be the 35th interruption) 00100010. Now when the preset or jam input is enabled the counter will reset to the starting point and when the count is enabled it will count down from 34 to 0. Then take an 8 input nand gate (4068) and connect it to the outputs of the counters. The output of the nand gate will change state once the count reaches 0 and can be used to drive an appropriate transistor which can operate your relay. A side benefit of this circuit is that the counter outputs could also be used to drive display decoders such as the 4511 and thus you could have a 7 segment LED display of the current count if it would be useful. If you try my idea and get stuck give me a shout and I'll try to help.