Hey all! I'm a complete electronics newbie, but I still try! Anyway, I need a little help. I want to put together a timer that works like this: 1. I close a switch 2. A timer delays 5 seconds 3. A second circuit closes after the 5 seconds. I looked at the schmitt trigger, but I cant figure out how to put everything together. Thanks!
So just to clarify, you close the switch on the timer, wait five seconds, then the other circuit switches on?
Yep, the main goal is to put lag in the button. Push...wait...ah... Releasing the switch can (and would be best) turn everything off.
I warned you that I was an electronics newbie. I know that pin 3 is often used on a 555 for the oscillation, so I am assuming that is is the "Output" that is referred to. But...is the output negative or positive? What is a common capacitor-resistor combination that I could most likely steal off some old projects? http://555-timer.clarkson-uk.com/operation/mono2.gif Thanks! Please do NOT hotlink to sites without their permission
The output will be low until the 5 seconds has passed then the output will switch high. Try look for 220 uF 20.66 K or 470 uF 9.67 K they seem to be fairly common.
There's a calculator on the page, input '5 secs' and he gives you a list. You can round the resistors off to the nearest standard value, so 220uF & 20k is one combination, 100uF and 47k is another. There's no point in being too exact on the resistor when a capacitor tolerance is +/-20%; if you want 5.00 seconds use a variable resistor and tweak. For what you say you want, the 555 output from pin#3 can be used to operate a relay to give power to the second circuit. If the relay isn't a low-power-coiled one you'll need a transistor switch on the 555, there are thousands of examples out there.
Isn't this just a schmitt trigger though? If I am going that way could I use the 74HC14 and accomplish the same thing? I've looked at using the 74HC14, but I figured using a 555 would be a lot simpler, I guess not... Thanks
You could make a crude delay with a Schmitt inverter, but you'll still need R1, C1, D2 and C3, plus you'll definitely need a transistor switch and its base resistor for current boost, so no simpler, less accurate, bigger footprint, higher cost. If crude will do, just delay a transistor switch-on.
The circuits shown here are both simple and reliable, so my post is just another way of doing it. You could also use a minimal microcontroller circuit(there are microcontrollers the size of a 555). This suggestion is more like an overkill for a simple timer circuit, but it could be a nice and simple start to get into microcontrollers(if you ever planned on doing that).
The only reason I ask about doing it with a schmitt, is that you can use smaller capcitors. I dont have anything about about 100uF, nor do I have (or can get within a week, im impatient) 10,000 Ohm resistors. There is no small way of doing this? A 470uF with 10,000 Ohms is going to be a fair sized capcitor. My 10uF are about as big as I can accommodate (size wise).
Sorry to piggy back onto this discussion but the new site layout is slowing my search down It's been a while since I last visited I want to create a similar timer that will turn off after 2 hours or so. I would like to use a switch to start the timer (then it would be inactive until it is re-pushed). I would prefer to have no power consumption until the button is pushed again. This timer would be used to turn power on to a charger and then cut the power after the alloted time. Hope this makes sense. Meblin
"no power consumption" means build it round an hour-glass or two , but you'll get very low power consumption from a CMOS 555 monostable plus MOSFET switcher, and I think it will handle a few hours delay with a low-leakage timing capacitor. Another way to get long delays is with a CMOS4060 chip, built-in oscillator and divide its frequency by 16384 to create a very slow astable; you'll need a system to stop it coming on again after 4 hours.