This timer is built on a 1.75" square board. As you can see there is room to spare.
Update 15 March, 2005: It's been some time since I updated this page, so I thought I'd throw in a few more details of how I'm using these timers now. The old information is at the bottom of the page, it contains the information on how to build the timer.
Here is a picture of a couple of timers I just made. They are the same, except the one on the right has a piezo buzzer that comes on when the timer is powered up.
The timing circuit is the same as I've always used, the changes here are adding the screw down wire terminals. The terminals make it easier to connect the deployment charge wires and the battery wires. These timers also use a transistor to provide more current to the igniters. The transistor I'm using is an IRL 520 HEXFET Power MOSFET, Digi-Key part number IRL520-ND. It's rated at 10 amps and works very well to deliver high amps to the deployment charge initiator.
I've been using a magnetic reed switch to power on the timers at launch. The magnetic reed switch is normally closed, only when a magnet is placed close to the reed switch does it open and not allow current to flow. So I place a magnet on my launch rail close to the reed switch, I know when the magnet is properly placed when the piezo buzzer on the timer does not sound. Of course, I also have a regular switch in line with the reed switch so the whole circuit is dead until the rocket is on the rail and ready to launch. It makes for a nice, clean setup. As there is no need for external switches, a gee switch or a break wire. The reed switch is part number CH403-ND.
The reason I now need the high amp version of the timer is because I'm using initiators made from ni-chrome wire. The 40 gauge wire requires about 1.5 amps to quickly light the thin layer of pyrogen on the tip.
One other thing I should note, I'm using a 500K ohm micro pot and a 100uF capacitor for long duration timers. This combination runs up to about 60 seconds, I use it for the main chute deployment in dual deployment situations.
On my software page I have a little program that calculates the values of the capacitor and resistor, giving you the approximate time-out time.
You can see the timers and magnetic reed switch in use on my Electronic Module 3 page. On that page I showed magnetic reed switch powering a relay. I decided to do away with the relay. The initiators I use never draw more than 1.5 amps, and the magnetic reed switch can handle 2 amps. So the relay isn't needed, though I do use two separate magnetic reed switches to make sure they handle the load.
End Update:
I wanted a timer to trigger my ejection charge. I wanted it to be simple, reliable, inexpensive and something I could make. So I began scouring the web for timer plans. Most were based on the 555 integrated circuit. So I based my search there. There were astable and monstable circuits, I figured out I needed a monstable circuit. So I narrowed my search. I found that most of the circuits were designed to be triggered, and start the circuit then turn off at the end of the countdown. I needed a circuit just the opposite, I needed the circuit to be triggered, then at the end of the countdown turn the device on. After hours of searching I found a couple of circuits that met my needs.
With only a few modifications, I had what I wanted. Here is a parts list, it's all available from Radio Shack.
.01 uF Ceramic Disc Capacitor
220 uF Electrolytic Capacitor
1N914 Silicone Switching Diode
555 Timer Integrated Circuit
47 K ohm PCB Mount Micro Potentiometer
General Purpose PC Board
9 Volt Battery Clip
Please note: The above diagram is looking at the circuit from the bottom side, the side you do all the soldering on, not the side with all the components. I have also been using an eight pin socket for the 555 chip. The sockets are real cheap and prevent heat damage to the 555 while soldering to the board.
Also Note: Pin one is identified on the 555 chip by a small notch or circle next to it. The package with the 555 will show the pin diagram as a mirror image of mine. That's because they are showing you the pin diagram from the top, I'm showing it here from the bottom.
Here is another diagram of the timer circuit. This is more of a real case wiring diagram.
The 555 timer is able to output a maximum of about 200 Milli amps, which is enough to fire the 2.5 volt bulbs I
use. But for higher amp use, a small PCB mounted relay may be used, or, you could wire a high amp transistor to
the output. The 220uF capacitor and the 47 K pot. control the time delay. In this configuration, the delay is adjustable
from the 47 K pot. from 0 to about 11 seconds. Longer time delays are possible by changing the 220uF capacitor.
Changing the 220 uF capacitor to a 470 uF should bring the maximum time up to about 20 seconds. You can also change
the variable resistor to a 100 K ohm. I typically use the 100K ohm now, and can still adjust to within .5 seconds,
and have a much longer delay.
In this design, the timer circuit is started when power is applied. That means a switch would need to be inserted in one of the wires from the battery. A simple lever switch with normally closed contacts can be used against the launch rod, or at the base of the rocket. Just so that when the rocket leaves the launch pad the switch closes and starts the timer. Another good idea is to put an arm switch inline with the launch detect switch, I also use a small piezo buzzer that is wired to the arming switch, just so I know when the circuit is active. Whenever there is a live deployment charge in the rocket, make sure you keep it pointed skyward, and away from anyone. Even if the circuit is not armed.
Keep in mind, if the circuit goes open for any reason, even for a moment, the timing will start over. So make sure you use a switch that will stay closed after liftoff.
Another method I use to start the timing circuit is a pull wire. Simply install a wire from the battery positive to the negative side of the 220 uF capacitor. When this wire is installed the circuit is active, but not timing until the wire is opened. With the pull wire installed and the timer running, the circuit only draws about 6 milli amps, that's not much and is not an appreciable draw on the battery. I have run the circuit for over an hour with no real loss of battery capacity. I run this wire down through the bulkhead and out the motor centering ring at the aft of the rocket. Then cut the wire to open the circuit, and tin the ends of the wire. Once on the launch pad, I use an alligator clip jumper lead that is wrapped around a bolt on the launch pad to again close the circuit. Then turn on the arming switch. When the rocket launches, it pulls free of the alligator clips, leaving the jumper lead on the launch pad and opening the circuit, starting the timer.
Always test your circuits before attempting a flight. Use a small light bulb or buzzer instead of the deployment charge.
