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Scratch - 1/2 Ounce Timer {Scratch}

Contributed by Jonny Dyer


This page describes the how to build a very small (1.5"x1.5", ~1 oz.) rocket ejection timer. I have tired to give as complete of instructions as possible, but you still might encounter problems. While it is possible to build this timer without printing a custom circuit board, I don't recomend it. In the past I have used the pre-printed circuit boards that mimic the layout of a breadboard, and trust me, they are much more trouble than they are worth. To ease construction of this timer, I am offering the following products:

1. Pre-printed and drilled circuit board
All you need is the components and a soldering iron ................................................US$12

2. Pre-printed and drilled circuit board with all electronic components
All you need is a soldering iron .................................................................................US$20

If you are interested contact me at


Fig. 1 - Circuit Schematic for Timer 



Fig.2 Component layout on PCB 



Fig.3 Copper trace layout for PCB. 

  • Q1, Q2 - Any good PNP switching transistor
  • S1 - Reed switch, break wire, etc...
  • C1 - 220uF Electrolytic
  • R1 - 100k Micrco Potentiometer
  • R2 - 1/4 watt 1k
  • SC1 - 200volt 6 amp SCR

This is a very simple, yet effective timer based on a basic RC charging circuit. It will time for an interval of about 3 seconds up to about 20 seconds based on battery voltage used and components used. R1 sets how long the timer runs. For longer times, a larger capacitor can be substituted for C1. Any voltage up to about 12 volts should work fine with this circuit, although I have only tested it up to 9 volts. S1 is the trigger for the timer to start. The timer will start when S1 opens (it is normally closed). I use a reed switch on the body of the rocket that matches up with a small magnet on the launch tower/rod. When the rocket moves away, the reed switch opens, starting the timer. As long as S1 is closed, Q1 is saturated, reverse biasing Q2 and turning it off. When S1 opens, C1 begins charging through R1 and Q1. When it is fully charged, there will be no more base-emmiter current and Q1 will turn off. This will allow current to flow through R2 to the base of Q2, saturating it and turning SC1 on. This fires the ejection charge.

Note that the load out on the PCB goes to ground when the circuti fires, so the ejection charge should be hooked up to the positive of the battery and the load output of the timer. Also note the oritentaion of C1 on the PCB and the orientation of the transistors.

As an interesting side note, I have been experimenting with low extremely lightweight ejection electronics and as such I am using a 3volt voltage source (as is shown in the schematic). I have found that the low voltage christmas tree lights, if broken open, will reliably light a BP charge on 3volts. This is how I plan to do ejection for Flogiston due to space and weight limitations.

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