An Interval Timer

Author: J.B. Hoag

Time intervals ranging from 0.001 to 0.2 second have been measured with the circuit of Fig. 30 O with an accuracy of 1 per cent. At the start, the gas-filled tubes 1 and 2 are biased by their C batteries below the striking voltage so that no plate current flows in either tube.

Fig. 30 O. An interval timer

A positive impulse on the grid of 1 starts its plate current. Shortly thereafter, a second impulse on the grid of 2 starts its plate current. The total current from both tubes, flowing through R, sets up a sufficiently large IR drop to lower the plate potential of 1 below zero and stops its current flow. The deflection of the ballistic galvanometer BG is proportional to the total quantity of electricity Q which surges through it during the time interval (t) between the two pulses (Q = It). The circuit locks out and cannot be excited by a second set of events until the plate current of 2 is turned off with a switch. The input pulses may come from photocell, microphone, counter-tube, or other pick-up device, so that a wide variety of timing applications are possible.