1,153,928. Tachometer, engine testing equipment. ALLEN ELECTRIC & EQUIPMENT CO. Aug. 30, 1967 [Oct. 12, 1966], No.39742/67. Headings G1N and G1U. [Also in Division G3] An engine testing circuit responsive to an electrical condition of the electrical system of the engine comprises a chargeable storage means connected across two terminals in the electrical system for producing a unidirectional potential and a parallel connected sensing circuit which senses the electrical condition and receives operating power from the storage means as a function of the electrical condition. For measuring the rotation speed of the shaft of an I.C. engine, the storage means comprises a first capacitor 56, which is maintained charged to the limiting voltage of Zener anode 57 by pulses produced in the ignition circuit of the engine whenever contact breaker 13 is opened. The sensing means comprises a second capacitor 34, charged from capacitor 56 to the limiting voltage of Zener diode 33, whenever a transistor 27 is rendered non-conducting, and which discharges through a D.C. meter 38 when the transistor is rendered conductive. The state of transistor 27 is under the control of a capacitor 43 which is charged each time the breaker 13 is opened to render the transistor conducting. A choke 51 is included in the leads connecting the tachometer to the ignition circuit to limit the effects of h.f. transients produced in the circuit when the breaker is opened.