Hey all, I'm thinking of building a timing circuit using a momentary switch that will start a circuit (a couple LEDs) and time out after 12 to 15 seconds. I'd rather not use a microcontroller for such a small project and have some ideas of implementation but am looking for some feedback. Does anybody have any good ideas of how to do this on the cheap?
You are in luck, I just finished buildin this project for my grandson. I just used an analog comparator to compare a capacitor charging through a series resistor to tthe center of two resistors in series. Made the little fella's eyes light up! The LED was on the output of the comparator. I hope this works.for you.
Google -- 555 timer circuits and you'll find no shortage of ciruits like this you can make with bascially a few parts, 555 timer, electrolytic cap, var resistor, and your 2 LED's, if you want to run more than a couple low power leds you can toss in a 2N2222 transistor drive some bright ones. You might check our quickkits or JamesCo or ElectronicGoldMine hobby kits, you can get a kit with the pcb for a few bucks, probably cheaper than you can by the parts for.
Alright, I think I'm going to go towards the 555 circuit as it's so incredibly simple and I found some lying about, though I think it wouldn't be too hard to go with a transistor/capacitor circuit for implementation. Thanks papalyle and WildBill!
Here is a simple 2 LED flasher: (You can swap the 220k resistor for a pot and make the flash rate adjustable.)
Here is a very simple 555 flasher circuit and you can use the table below it to get your frequency and part values: (This will work from 3 - 12VDC)
555 astable frequencies
R2 = 10k
R2 = 100k
R2 = 1M
Use a LMC555 and the circuit of Fig. 5 at www.ti.com/lit/ds/symlink/lmc555.pdf for a single pulse each time the switch is pressed (monostable operation). Be sure to connect the reset pin to the + power supply. The trigger input also needs a pullup resistor to the + supply, and the switch should take the trigger input to ground. If the switch might be held down longer than the output pulse, you will need to couple the switch through a capacitor and provide another resistor to discharge the capacitor when the switch is released. A LMC555 is better than the older LM555 because the LMC555 has an output that is essentially rail-to-rail. A square wave oscillator (astable operation) with a LMC555 is simpler than shown in Fig. 8. of the TI document. All it takes is one resistor from the output to the threshold and trigger inputs, tied together, and a capacitor from this point to ground. The discharge input is not connected to anything.