How to Make Electronic Timers Using the “555” IC
This is actually two separate circuit plans – #1 – a timer which triggers a relay, and #2 – the relay controlled ignition circuit. After looking at the schematic, you may wonder what the point of the relay circuit is – why not just put the igniter in place of the relay coil on circuit one? The answer is this: There is not enough current to burn the steel wool, due most likely to the resistance of the 555 timer, and maybe other factors… I’m not sure, I just know that the steel wool doesn’t burn. So, the second circuit is completely independant of the first one, except for the relay which switches the current on after the timer is triggered. Therefore, you will need two nine volt batteries for this circuit to work.
You can buy these at a electric parts store, or salvage them from garage junk, or whatever. DON’T buy them from Radio Shack – The 555 timers cost $3 EACH, and you can buy them for about 55 cents elsewhere. (ask the electronics teacher in your school, or look in the phone book)
- S1,S2 – Any working Switch – whatever you can salvage
- C1 – any value capacitor (keep reading for info)
- R1 – Resistor, value depends on desired delay (keep reading for info)
- IC1 – 555 timer
- Two 9V batteries
- Small Relay (grab it from one of those 100 in 1 electric kits, or buy one)
- IC socket
- PC Board kit (would really help – or use Perfboard)
- Wire, Solder, Solder pencil, Electrical tape, etc…
Just in case you’re not sure, the pin numbers on the IC (555 timer) are as follows:
(make sure you don’t accidentally do what I originally did – which was to think that pin #8 was number 5, 7 = 6, 6 = 7, 5 = 8 – that seems more logical, so it’s easy to forget when soldering)
To determine the value of resistor and capacitor to use for your desired time, use the following formula:
Time (in seconds) = 1.1 x R x C
Therefore, to obtain a time of 15 seconds you would:
1. Find a good quality capacitor from junk you have laying around. (in this example, say it’s 50 aeF (micro-farads))
(*** IMPORTANT: change Micro-farads to farads first – divide by 1,000,000)
(pF – pico-farads are 1,000,000th of a aeF (micro-farad) – probably too small for this circuit)
2. Determine the resistor value to use:
Time = 1.1 x R x C
15 = 1.1 x R x 0.000050
R = ————–
(1.1 x 0.000050)
R = ————–
R = 272727.27 ohms
Therefore, for an approximate 15 second delay, you would need a resistance close to 272,727 ohms. (272k)
(when I wired this up, I wanted approximately a 10 second delay, so I used a 50K resistor. Before adding the relay, the time was 13 seconds. After adding the relay & shit, the time decreased to 7 seconds. I have NO idea why. Just keep this in mind. Maybe it was pure luck in my case, or bad soldering. I don’t know…)
3. Find a resistor near the determined value, and solder your circuit up (using wires, a Printed Circuit Board, Perfboard, or whatever you have around).
To aid you in salvaging resistors from old parts, I have decided to describe the color band system of marking resistances on resistors. If you know all this already, skip to #4.
Reading resistor color bands:
1st band = 1st digit in number
2nd band = 2nd digit in number
3rd band = Number of zeros after first two digits
4th band = Tolerance (ALWAYS Gold or Silver)
To read, make sure the gold or silver band (sometimes looks greenish) is on the right.. Then the numbers are as follows:
Black = 0 The final band is either Gold or Silver…
Brown = 1 These are tolerance levels and aren’t really
Red = 2 important for this circuit…
Orange = 3 Silver = 10%
Yellow = 4 Gold = 5%
Green = 5
Blue = 6
Violet = 7
Remember this as BB ROY G BV… Trust me, it helps once you have memorized this.. You can remember which B is brown and which is black since Black is the absence of color, therefore black is 0…
Now an example:
You find a resistor in your junk box with the following colors:
1st – Red 2nd – Red 3rd – Yellow 4th – Gold
Since Red = 2 and Yellow = 4 this resistor is equal to 22 followed by 4 zeros or 220,000 (220K). The gold band is the tolerance which is 5% (most common).. This means it could be 5% off of the marked value, so the resistance of this particular resistor will fall between 209,000 ohms and 231,000 ohms.
4. Attach the positive terminal of a 9V battery to one wire of a switch (not shown on Schematic diagram), and the two positive inputs on the circuit to the other wire of the switch. This is your “ARM” switch. Mark it as such. When this switch is off, there is no way in hell the thing will go off, but to be extra cautious, you might want to add another switch in series with the relay (switch part) and the other nine volt battery. This will be a bit of a pain though (putting three switches on the device is just asking for trouble/confusion), so don’t do it unless you really are paranoid, or are building a pipe bomb big enough to blow up the world trade center.
5. Now, the fun part. Find a suitable container (I used the top half of a plastic 1.25 litre 7-UP bottle, and the bottom hard plastic thing as a removable cover to change the batteries), and make two holes for the switches, sized and placed according to what the switches you used are like. Make sure you mark which way is on. Turn both switches off, and fasten into place with the movable part sticking through the hole. Mark them as “ARM” and “TIMER” with tape or something. Now, everything should fit into the container. Make two small holes for your igniter leads, and feed them through. Place the circuit in the container, put the two batteries in their clips, and close the container. You now have a portable timer system. To use it, FIRST turn the “TIMER” switch ON. AFTER, and ONLY AFTER the “TIMER” switch is ON, turn the “ARM” switch ON. Now, as soon as you turn the “TIMER” switch OFF, the countdown will begin. Run like hell! This sequence may seem a bit strange, but believe me, it’s necessary. Just try it using an LED (light emitting diode) and you will see it works. Once again, the sequence is: (You should clip & print the text below, and tape it to the device)
Keep Both switches OFF at all times except when using. When ready to use:
#1: “TIMER” turn ON.
#2: Connect bomb (ARM is OFF and TIMER is ON)
#3: “ARM” turn ON.
#4: “TIMER” turn OFF. (starts timer)
#5: Run like hell.
That’s it. You are now the proud owner of you own, home built, electrical timer. Just make sure you always use fresh 9V batteries and it will rarely, if ever, fail.
The maximum resistor you can reliably use is approx. 3.3 Mega-ohms (3,300,000 ohms). Using an R1 value of 3.3M ohms, and a C1 value of 0.5 Farads, you could construct a device which would go off after 1,815,000 seconds (1.1 x 3,300,000 x 0.5), or 30,250 minutes, or 504 hours, or 21 days! 3 weeks! If you were sure of what you were doing, you could construct an explosive which you could leave somewhere 3 weeks ahead of the time you want it to go off (assuming your battery doesn’t run down first!).
The accuracy would probably decrease depending on the time you wish to use, so you can only make it go off at a general time (ie: you couldn’t make a device go off at exactly 12:04, 17 days from now, but you could most likely make one to go off at approx 12 noon tomorrow, and you could definately make one go off in almost exactly 45 seconds))
*** VERY IMPORTANT ***
The entire length of steel wool of the ignitor absolutely MUST be in contact with the black powder. Otherwise you may have a misfire.
If you happen to have lots of money and nothing to spend it on, you could buy electrical ignitors for model rocketry (called “Solar Ignitors” for some stupid reason). These would be more reliable and better for those devices where misfires would really piss you off.