This article appeared in an old issue of High Power Rocketry magazine. It is the timing circuit that was used for my level 2 camera rocket, the Nike 2000.

Just another Timing Circuit
by Douglas Gerrard
TRA #568

This camera circuit, developed for the Nike 2000, is a multi-functional timing circuit designed to take pictures at takeoff, separation, and upon descent for cameras with electronic shutter controlled mechanisms. The camera looks out the side of the rocket and through a mirror looking down. Since the camera can be shot in a continuous mode, this circuit will take pictures continuously from takeoff to two seconds after separation or at a slower rate if desired. This article will give you the schematic diagram and circuit board layout for you to construct this circuit board for your own camera.

Features include circuit board mounted switches in parallel with ceramic chip resistors to adjust the time in between pictures. It is designed to be used with Adept Rocketry's® acceleration switch so there is no need for external switches to be activated from a launch lug or a pull out plug.

The circuit design (Figure 1) is composed of three separate circuits on one circuit board. (1) The Takeoff circuit takes pictures from liftoff to separation, (2) the Separation circuit controls how many pictures are taken in continuous mode after separation, and (3) the Descent circuit controls the initial delay after separation and the time interval between pictures on descent. With the use of IC chip holders any portion of the circuit board can be de-energized by simply removing the 555 chip. Switch S1 is the Master ON-OFF switch. The position of switch S2 controls that portion of the circuit board is energized. S1, S2, and S3 are mounted on the rocket and not on the circuit board. Note in the schematic diagram that several resistors are shown as variable. They are not potentiometers. These resistors are in parallel with the circuit board mounted switches.

Take-off Circuit. With switch S1 ON and switch S2 depressed, power is available through the normally open (NO) contact of S2 to energize the acceleration switch. After about a half second after takeoff the acceleration switch energizes the Takeoff circuit. The time in between pictures is controlled by capacitor C4 and resistors R14-R16 and the circuit board mounted switches one (CBS1). The position of CBS1 shorts resistors R14 or R15 to change the time delay between pictures. With both of CBS1 switches ON the camera will shoot in continuous mode. Most SLR cameras will take a picture every 0.5 seconds in continuous mode. With either switch of CBS1 OFF, pictures will be taken about every 0.67 sec (3 pictures every 2 seconds) or 0.75 sec (4 pictures every 3 seconds).

Separation and Descent Circuits. Once switch S2 is repositioned at separation, its NO contact de-energizes and the takeoff portion of the circuit board and its normally closed contact (NC) energizes the separation and descent circuits. The purpose of switch S3 is to momentarily short the 555's pin 2 to ground to start the timing cycle. Resistor R10 is a pull-up resistor to prevent inadvertent triggering. Switches S1, S2, and S3 are mounted on the coupler of the payload section but S3 must be mounted below S2 (Photo 1). Switch S3 must be momentarily shut after switch S2 repositions.

The number of pictures taken at separation in continuous mode depends on capacitor C3 and resistors R11 and R12. CBS2 determines if 1, 2, 3, or 4 pictures are taken at separation (i.e. the relay is shut for &Mac189;, 1, 1&Mac189;, or 2 seconds). Capacitor C2, resistors R7 through R9, and CBS3 controls the delay time after separation for the descent circuit. This allows the payload to settle down after the parachute deploys. Capacitor C1, resistors R1-R6, and CBS4 control the delay in between pictures on descent.

Figure 2 and Photo 2 show the component layout and Figure 3 shows the location for the surface mount resistors on the bottom of the circuit board. Table 1 lists the required components. Most are available from Radioshack®. You choose the values of the resistors and capacitors to suit your own camera and needs. Radioshack® has a very nice handbook by Forrest Mims III about the 555 timer to assist you. Use large capacitors (200 mF), which require smaller resistances for the desired delay times. This results in more stable times. (Stray capacitances in the circuit board or stubborn dirt and flux under the chip resistors has less of an effect on the delay times). Use large switches for S1, S2, and S3 such as switches for refrigerators that turn on the light or turn off the motor. They are very reliable. Micro-switches can break or not activate when they're supposed to due to the tight tolerances. Refrigerator switches can have up to &Mac189;" throw and never seem to wear out. If you do not want to use Adept Rocketry's® acceleration switch, connect a normally closed switch to the connector at pins 1 and 2. This could be depressed by the launch rod and would start the camera at liftoff. Note that the black wire from pin 1 does not get soldered to the circuit board but rather goes straight to the normally open contact on switch S2.