I have a nifty little mod idea but I've run into a snag. Without revealing the details or anything, I was hoping I could get some help. Basicly, I want a relay (I assume thats what it is) to act as switch when an LED lights. On whent he LED is on, off when the LED is off. Now, the only multimeter I have is older than I am and dead, so no info on the LED. It's a 5mm and probably around 2-3.5 volts. As for the resistance, no clue. Here's what I need to know: A: What kind of relay (or whatever) do I need? B: Will such unit be able to be used as a power switch for a computer(I'm sure it'l work, just wanna make sure there is no risk of crisping up my mobo)? C: What would I need to make sure that the relay is a only on for a maximun of 2 seconds even if the LED is on?
A. Hard to say without you telling us more, but you most likely will need a transistor to provide enough current for the relay to switch, and a diode in parallel with the relay to prevent back EMF. B. Yes. Be aware that the process of turning on the computer using the power switch header is briefly connecting the two contacts together. C. Can't think of anything definite off the top of my head.
That's possible, but it'd require much more complex circuitry. Perhaps if you posted some details of what it is that you're trying to do, people would find it easier to come up with a solution
probably a 555 timer... yeah, we won't take you idea, till after u use it amyway... plus, its easier to help, and, since were helping, we have a tiny right too know what it is, out of curiosity.
Sorry bout the previous lack of info. What I want to do is use this key pad (pics below) to start a computer. It controlled the lock for a cheap safe. While I'm not sure exaclty how to accomplish it, I think that it can be done. Well, after studying the various behaviors of the unit, I figured I keep it simple. The green (open) LED only lights after the correct combonation has been entered. So I wanted to relay off of that. The problem was, the light stays on too long and would keep the relay open too long as well (roughly 8-9 seconds). So, that added the need for a timer so make sure the relay is open just long enough to boot the machine. I'm betting there is another way to accomplish the whole thing. One of the big problems here is I have only some really basic circiut construction know how. This is the back of the unit with four parts attached. The large grey piece to the left is the battery cage for three AA's in series. Below the main keypad in the somewhat blacked out area is a switch the tells the main unit when the safe door is open. When press, the switch opens the circuit telling the unit the door is closed. When not press, the circuit closes indicating an open door. The round part just to the right of the door switch is a speaker. Next to it is the mechanism the jamed the door. When the door was shut and correct combonation not entered, a spring forces a small rod out keeping the door know from rotating (I can resassemble the lock and take pics if needed). When the correct combonation is entered, the electro magnet inside the part pulled the rod back against the spring, thus allowing the door knob to rotate. Once the the door switch told the main unit that the door was open, the magnet would release. It would also release when the LED went out after the 8-9 seconds. Here is the front of the pad. Summery of unit actions: -4 digit combonation using 0-9. -Incorrect PW entered- "Closed" Lights twices with beeps from speaker and flashes quickly 10 more times with no noise. -Correct PW entry activates electro magnet allowing safe door knob to rotate. "Open" lights with noise from speaker. "Open" LED and electro magnet shut off after 8-9 seconds without action. -Open door after correct combonation entry- Electro magnet immediatly shuts off re-engaging locking mechanism. "Open" LED shuts off in 8-9 seconds after door switch open. -Door open indicator tells main unit door is open at any time- "Open" LED lights with door noise from speaker. LED light shuts off after 8-9 seconds. I think thats the gist of it. If I can, I'll try to pick up a multimeter so I can get current info.
An NE555 monostable timer with its trigger input connected to where the LED currently is would do the job fine. One that looks suitable to do the job is here: http://www.temp.eleinmec.com/issue4.htm Whilst it is shown lengthening a short pulse in the diagram, the same circuit should also shorten a long pulse, like what you are trying to do. To operate the Relay you need as an output, a simple transistor driver can be used. (connected to Pin 3 of the NE555 chip via a resistor) To make it work correctly, the NE555 circuit will need to share the same ground as the lock's circuit. If the lock uses a power supply between 4.5 and 15 Volts, it is probably easiest to let the timer cirucit share the power supply from the lock board. (Looking at the photo, the lock appears to use 4.5V - it may be 'borderline', in which case there may be problems. Since you are planning to use this in a PC, you may be able to run it off the 5V standby line, so you won't need batteries, which would also mean that you could definitley run the NE555 off the same supply)
