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I need to make an electronic timer. Answered

I want to use a electronic timer to set off a buzzer. LED or vibrating motor, or to ignite things, how can i do this?


The simplest way of making a timer is to use a 555 timer IC. This only needs a few peripheral capacitors and resistors to create a monostable timer.
The schematic below is a typical circuit. The time constant in seconds is 1.1RC.
Polystyrene, polycarbonate or ceramic capacitors should be used rather than electrolytics which tend to have high leakage.
The datasheet for the 555 timer is available here:

The operation of the timer is as follows:
When the trigger input falls below one third of the value of the supply voltage, 12/3 = 4 volt in this case, the output activates for a time determined by the time constant. The trigger input could simply be pulled low with a push button to achieve this. The output can either source up to 200 mA into a load or sink 200 mA from a load tied to the supply line.


I almost forgot: Regarding questions like, "Which wire from the piezo do I use?", "What size resistor?" "How come (whatever) doesn't work?" If you're adventurous enough to try this you should also have the initiative to figure out some things by yourself. Getting this thing to work, for your particular whatever, will take undoubtedly take some experimentation to make it work. Also the diagram given here is just a suggestion. No warranty, or fitness for a particular purpose is expressed or implied. Plus the usual caveats about how YOU, not me, are ultimately responsible for YOUR own safety, sanity, credit-rating, etc.

I am guessing you already have an electronic timer, e.g. a kitchen timer, and you want a circuit to sense the alarm signal and change it into different signal, like closing a transistor, or a relay, for the purpose of switching larger currents, to actuate your LED, or vibrating motor, or coffee machine, or ultimate terror weapon, or whatever.

The alarm signal to the piezo transducer of the kitchen timer is mostly AC (when the alarm is sounding). That's the signal that is there. The signal you want, for the purpose of turning on your other thing, whatever that thing might be, is likely a DC signal, or something close to this, staying "on" for several seconds.

If this is the case, then the problem electronically speaking is one of "detecting" or "rectifying" an AC signal into a DC signal. The usual trick (assuming the signal present is greater than 1 diode drop in magnitude) is to use diodes and capacitors. Circuits that do this have names like "half wave", "full wave", "doubler", "multiplier", etc.


It's been a while since I built anything like this. Please note that the diagram I've attached to this comment is untested, but think it *might* work. For this hypothetical creation, I am assuming that the original unmodified kitchen timer runs on a single cell (1.5V) supply, and that I can just stack a few more cells on top of that to power the relay coil and LED indicator. One of the wires that used to go to the piezo transducer, now goes to the input of a Greinacher voltage doubler. (I didn't know it was called that either, see the wiki article on voltage doublers.) The output from the voltage doubler goes, through a resistor, to the base of an NPN transistor. When the alarm sounds, or maybe even when it just beeps a little, the AC signal, that used to go to the piezo, now charges up the capacitors in the doubler. Then these lean on the base of the NPN transistor, providing current to turn it on. This in turn, switches on the relay and the LED indicator. One of the things I like about this doubler circuit, is that it should produce zero response to any DC signal (since it has a capacitor in series with its input).

Now here's the most important part: if the accessory thingie you are switching with the relay is the least bit dangerous, you should put an "arming" safety switch in series with the relay contacts. The reason why is that the beep-beep-beep noises the timer normally makes while you are setting it will likely also trigger the relay, just like the alarm does. So then while setting the timer and thinking, "Hmmm... How much time do I need to get away from this dangerous thing. Then some beep-beep button pressing, and the relay closes and some deep irony ensues! That's because there was no time delay, and you're right on top of the thing you wanted to get away from, thus defeating the whole purpose of building a timer. The LED indicates when the relay coil is energized. Closing the "arming" switch while this LED is on, is also "bad".


you could use a 555 timing circut! just google it and lots of sites will come up!!

. Use the search widget near the top of most pages. If you don't like the results, use Google or your favorite web search engine.

I've tried, it a;; seems to be for appliances.

. When you read the iBles, substitute "buzzer. LED or vibrating motor, or to ignite things" for appliance. It's just a switch closure.

i tried google, didn't work either.