Need help with this circuit - Page 1
Forum section: electronics / beginners

author: darinsquared
author_rank: Contributor
timestamp: December 06, 2016, 09:56:09 pm
content: I want to make a lock very similar to the one on this site:http://www.allaboutcircuits.com/textbook/experiments/chpt-7/simple-combination-lock/The part I do not understand is how do you make a tactile switch or keypad remain "connected" to maintain current once you release it to enter a second digit? Would adding a capacitor store enough current to maintain voltage when enter remaining digits?

author: Benta
author_rank: Super Contributor
timestamp: December 06, 2016, 10:41:47 pm
content: That circuit does not use tactile switches or keypad. It uses static switches.

author: Brumby
author_rank: Unknown
timestamp: December 07, 2016, 12:51:56 am
content: The switches are used to provide a binary value ... the only 'digits' you can enter are 0 or 1. That circuit has only 16 possible combinations - and one of those will be the rest state, so you would not want that to be your combination!You could use tactile or other momentary switches for the 'code entry' panel, it's just that you would need to press all the ones that needed to be on - at the same time. The 'lock' would remain 'open' only while they are held. However, you could not use them for the 'code set' switches. You would have to use a method where the code could be set and the values held. The DIP switches are one way. Jumpers are another.

author: darinsquared
author_rank: Contributor
timestamp: December 07, 2016, 01:22:39 am
content: how do security code readers work such as PIN's at ATM's where you enter each number individually?

author: mash107
author_rank: Contributor
timestamp: December 07, 2016, 02:16:44 am
content: usually a 16-bit encoder (MM74C922 or similar) to take a row & column ID and convert it to a 4 bits corresponding to the positionDecent article that gets into encoding, debouncing (preventing a single key from registering multiple times), etc:http://embedded-lab.com/blog/mm74c922n-based-encoded-matrix-keypad/

author: Luminax
author_rank: Regular Contributor
timestamp: December 08, 2016, 08:01:22 am
content: Quote from: darinsquared on December 07, 2016, 01:22:39 amATM Machine PIN pad is already into the realm of microprocessing and most likely are just QWERTY's Numpad parts read into the ATM computer, kind of not what you want to compare against if you want to build a simple key combination circuit.The circuit in the link you posted works in a "Compare A state to B state" style of approach where the circuit compare what switches are on at the A side (Key Code) compared to B side (Entry) and will give green or red after you push the enter key depends on if it matches or not.As mentioned by Benta, the switches in this practice circuit is supposed to be DIP switch bank that 'locks' into place.If you're looking into the wonder of "Press and Hold" using simple push switch, I think it goes into the realm of sequential logics with Flip-Flops and buffer and all the wonders of the digital world

author: salbayeng
author_rank: Frequent Contributor
timestamp: December 08, 2016, 09:13:03 am
content: The circuit in question is only intended for educational purposes , if you used a thumbwheel switch it would take a few seconds to rotate the thumbwheel once and open the lock.It might almost be practical if you duplicated the circuit 4 times and had 4 thumbwheels, and a button "press to open" that was capacitively coupled. But thumbwheel switches are $10 each!!.

author: Luminax
author_rank: Regular Contributor
timestamp: December 08, 2016, 09:57:53 am
content: The question now is, do you want to try to make a lock like the circuit mentioned to learn about the circuit?Do you want to make a combination lock and that circuit is one of the starting point and you want to go further?either way, first of all we need to make the goal clear as that helps a lot

author: iaeen
author_rank: Regular Contributor
timestamp: December 08, 2016, 02:47:15 pm
content: On the subject of "what are the real goals here", if you are interested in creating a real lock to be used for real security, you will need to think about hardening it against attack. Nevermind the fact that this design would only have 16 combinations, if an attacker can simply open it up and see the combination on the second set of switches (or bypass the logic altogether with their own external voltage source), the lock has failed.Real electronic locks need to put more effort into hardening than they do into their core locking functionality. See EEVBLOG #942 for an example of what this looks like.

author: mc172
author_rank: Frequent Contributor
timestamp: December 08, 2016, 08:24:16 pm
content: The way you make the tactile switches latch is with the use of a fair amount of additional circuitry which you'd have to repeat for each separate switch.There's plenty of exampleshere.Check out EEVblog #762 and #771 as well.