ring counter - Page 1
Forum section: electronics / beginners

author: tlovie
author_rank: Contributor
timestamp: December 24, 2022, 08:34:41 pm
content: I'm trying to design and build a ring counter out of discrete components. I have found a few blog posts - but not much that describes the operation in detail. So far I have found this circuithttps://hackaday.io/page/12672-circuit-alan-yates. Based on my limited understanding of how this works, is that to the right, you have a biasing circuit for the 3 transistor SCR things, and you have some capacitors that somehow link the stages, and the clock on the left, essentially cuts the power rail to cause the stages to advance.What I wanted to try is to make a similar circuit with discrete SCR components (BT169) to reduce the part count some more, as shown below. I've built up the circuit on a breadboard with about a 1Hz clock input, as you increase the bias voltage from zero to about 0.6V, all the LEDs glow very slightly. My thinking here is that the SCRs are conducting slightly through the gate, but not enough to latch on. As we get to about 0.8V, the behaviour changes, and only a single LED lights, and it appears to me that the SCR is in a fully latched state. When this happens, the voltage drop of R4 causes the voltage at the cathodes of the SCRs to prevent any other ones to latch. If we increase this bias voltage further, all of the SCRs will latch, but this is not what I'm going for - so I've concluded that the correct way to bias the circuit is about 0.8V. So I have the situation where one of the LEDs is lit - lets assume D1, and the other ones are out. This would lead me to believe that C3 would charge, and C1 and C2 would not charge. Here is where I'm kinda stuck - as the circuit continually only flashes D1, and it never cycles.What I would like to understand better is how this circuit is supposed to work in the original post, and if there is any remedy to get my circuit working. I know that I could get this to work with flip-flops, or a microcontroller - but I wanted to understand how to do it the old-skool way. I'm trying to make a divide by 3 counter - is there a better way to do it?Thanks,Tom.

author: jwet
author_rank: Frequent Contributor
timestamp: December 27, 2022, 05:53:30 am
content: I'll try to explain this circuit, at the end of your post, you say that what you're really after is a divide by 3 circuit, this is lot of trouble for a divide by 3 divider. That can be done with two flip flops and a couple gates- I could draw it but I'm sure that Google will do a better job- let me know if not and I'll post something.The idea of a ring counter is to have each stage held in a quiescent untriggered state at power up. Usually there is a reset circuit that insures this. When a trigger comes in, it triggers the first stage on, the each stage is some kind of latch, it turns on but doesn't carry on - whether its built with cross coupled transistors or SCR's (you're wise to see the equivalence). The stage should trigger but not propagate the trigger until another pulse comes in which triggers only the next stage etc. Finally at the termination of the last stage, there is a wrap around that resets them all back to quiescent. Doing this discretely this way is pretty fiddly, it depends on bias levels and feed through which you've probably observed. You can get it to work but its pretty tweaky.In the old days- like the 50's, there were circuits like this that used neon lamps, They behave a bit like SCR's, latching, etc. Fun to play with but not so practical.A couple of flip flops and some gates will do a better job or a single chip like the 4017 ring counter might be simpler.Enjoy.