In a circuit for operating a solenoid S for a predetermined time, if the duration of an input pulse lies outside certain pre-set limits, a positive-going leading edge is initiated by wire 30 leaving pin 40, which causes transistor Q1 to conduct, thereby cutting off Q2, Q3 and Q5. Capacitor C1 charges through R1 and C2 through R2 but at a slower rate, while Q3 removes the short across thyristor SCR-1. If the pulse terminated by 30, 40 reconnecting is of the correct duration, Cl will have charged sufficiently long for the potential of the emitter of unijunction Q4 to have risen enough for it to conduct, and feed a firing pulse, to SCR-1, the fall in anode potential allowing substantially complete discharge of C1 over diode D1 As a result, when Q2 conducts again, the pulse developed by C1 discharging through R3 is insufficient to fire SCR-2, while the conduction of Q3 quenches SCR-1 at its anode. If the pulse terminates before the pre-set limit, Cl will not have been substantially discharged from Q2 conducting, the pulse developed across R3 is effective when fed over D2 to fire SCR-2, causing S to operate. If the pulse length exceeds the preset limit, C2 will have charged sufficiently for Q6 to conduct and feed a pulse over D3 to fire SCR-2. Limitation of duration of operation of S. In the normal state SCR-3 conducts, the low anode potential maintaining mean Ne extinguished. When SCR-2 fires, the fall in anode potential, applied over C4, quenches SCR-3. C4 now charges over R8 until Ne ionizes, whereupon SCR-3 fires and extinguishes SCR-2 so that S releases.
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