deciding between two BCD signals - Page 1
Forum section: electronics / beginners

author: carbon dude oxide
author_rank: Frequent Contributor
timestamp: January 09, 2013, 10:49:13 pm
content: hello.im currently making a clock but i do not wish to have a micro controller in it. i have two BCD signals, one for the clock and the other cycles threw 0-9 rapidly what i am trying to do is switch from the clock to the cycle BCD for 10 seconds then return back to the original BCD input. How would i do this without relays as i would like it to be silent(by relays i mean 2 DPDT to switch from 1 BCD input to the other and then back again)any help would be brilliant thanks

author: baljemmett
author_rank: Unknown
timestamp: January 10, 2013, 12:22:25 am
content: Presuming that your BCD signals are at a sensible logic level, something like the 74*157 would do most of it -- the '157 is a quad 2-input multiplexer, so it takes two four-bit inputs and outputs one or the other depending on the state of a 'select' input line. Then you'd just need to drive that input with a 10-second pulse, which you could generate however you like -- for instance, with the old favourite 555 timer.

author: carbon dude oxide
author_rank: Frequent Contributor
timestamp: January 10, 2013, 04:44:09 pm
content: thats brilliant thanksalso how would i make a 555 timer do a 10 second pulse?

author: SeanB
author_rank: Super Contributor
timestamp: January 10, 2013, 05:09:41 pm
content: If it is a clock why not use a TTL counter and drive it off the 1Hz clock that you must already have there. Plenty of TTL decimal or hex counters to choose from. Even a 4017 will work.

author: carbon dude oxide
author_rank: Frequent Contributor
timestamp: January 10, 2013, 10:23:20 pm
content: Quote from: SeanB on January 10, 2013, 05:09:41 pmIm going to be using some 74HC390 chips to drive my clock but because there nixie tubes and not LED displays they can get cathode poisoning so to get around this without using a programmable IC im going to have 1 set of BCD for each tube to control the time and this will be on a 1Hz pulse ect and to prevent cathode poisoning i am going to make the digits cycle through its number repeatedly for 10 seconds which is where the second lot of BCD's come into play so on the hour i need it to switch to the cycle bcd and then after 10 seconds switch back to the clock bcd.

author: SeanB
author_rank: Super Contributor
timestamp: January 11, 2013, 05:56:16 am
content: So all you need is a 1 minute pulse to drive a flip flop and then reset it from the output TC of a BCD counter, that way you have no timing issues with the capacitor, and the BCD counter can be reset with the same 1 minute pulse as well to count up consistently. Depending on the rest of the circuit you will need 2 or 3 IC's for this, or use spare gates from other logic, when you will only need the BCD counter.

author: carbon dude oxide
author_rank: Frequent Contributor
timestamp: January 11, 2013, 07:32:35 am
content: It needs to be a 10 second pulse and what is a flip flop (still learning termanoligy)

author: SeanB
author_rank: Super Contributor
timestamp: January 11, 2013, 08:06:34 am
content: RS bistable. The 1 minute is the pulse used to clock the minutes from the seconds side divider. Very short duration pulse. You use this to reset the bistable that changes the outputs to the BCD counter from the regular counter, and the reset for the RS bistable comes at terminal count from the BCD counter to show the time again.