R/C Timer-Switch for Radio Control Applications

Posted on Feb 7, 2014

The project started with a request from a fellow member of the Long Eaton Club, Tony Bowler. He was interested in converting one of the `Golden Oldies` to electric free-flight. He choose Vic Smeed`s `Debutante` to power with seven AE size cells and a Speed-400 motor, but realized he required some sort of timer to give a predetermined motor run, an

R/C Timer-Switch for Radio Control Applications
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d asked if I could help. The design I came up with seems to fit the bill. It will switch up to 12 amps, has an optional soft-start for gear box applications, is simple to construct and all the bits are readily available from Maplins for under $10. The `ON` time is adjustable from 20 to 80 seconds, but other variations of the running time can be achieved. The heart of the circuit is the popular CMOS Timer IC, 7555. When power is applied from the battery, the IC`s output pin 3 is switched `OFF` (0V) and a short circuit applied to C5, thus ensuring it is completely discharged. When a momentary start signal is applied to pin 2, the output immediately switches `ON` (+V), thus switching on the MOSFETs and the motor via the soft start circuit R4, D1 and C6 (if fitted). This latter circuit slows down the switching on of the MOSFETs, giving a more gradual increase in current to the motor. If soft start is not required, omit these components and wire pin 3 directly to the MOSFET gate terminals G. When the voltage across C5 reaches 2/3V, the IC detects this and its output switches `OFF`, thus immediately switching the MOSFETs and the motor off via D1. The 7555 timer IC also re-applies the short across C5 and the circuit returns to its initial condition ready for the next motor run. Zener diode D2 provides protection to the MOSFETs from the high frequency transients that all DC motors generate, but do not omit the normal spark...

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