Dried Seed Timer

Section VI

No. 6

A time delay device for electrical firing circuits can be made using the principle of expansion of dried seeds.


  • Dried peas, beans or other dehydrated seeds
  • Wide mouth glass jar with non- metal cap
  • Two screws or bolts
  • Thin metal plate
  • Hand drill
  • Screwdriver


1. Determine the rate of rise of the dried seeds selected. This is necessary to determine delay time of the timer.

a. Place a sample of the dried seeds in the jar and cover with water.

b. Measure the time it takes for the seeds to rise a given height. Most dried seeds increase 50% in one to two hours.

2. Cut a disc from thin metal plate. Disc should fit loosely inside the jar.

NOTE: If metal is painted, rusty or otherwise coated, it must be scraped or sanded to obtain a clean metal surface.

3. Drill two holes in the cap of the jar about 2 inches apart. Diameter of holes should be such that screws or bolts will thread tightly into them. If the jar has a metal cap or no cap, a piece of wood or plastic (NOT METAL) can be used as a cover.

4. Turn the two screws or bolts through the holes in the cap. Bolts should extend about one in. (2 1/2 cm) into the jar.

IMPORTANT: Both bolts must extend the same distance below the container cover.

5. Pour dried seeds into the container. The level will depend upon the previously measured rise time and the desired delay.

6. Place the metal disc in the Jar on top of the seeds.


1. Add just enough water to completely cover the seeds and place the cap on the jar.

2. Attach connecting wires from the firing circuit to the two screws on the cap.

Expansion of the seeds will raise the metal disc until it contacts the screws and closes the circuit.

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