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Tuesday, May 1, 2007

RC Switch for Radio Control

Parts List
All resistors 1/4w / 5% tolerance, unless otherwise posted.

R1 = 47K
IC1 = MC14013B
R2 = 1K
Q1 = BUZ11, IRFZ42, NTE2395, or ECG2395
R3 = 10K
S1 = on/off switch (optional)
P1 = 100K

Servo Lead
C1 = 22nF

The MC14013B dual type D flip-flop is constructed with MOS P-Channel enhancement mode devides in a single monolithic structure. Each flip-flop has independent Data, (D), Direct Set, (S), Direct Reset, (R), and Clock, (C), inputs and complementary outputs (Q and Q-not). These devices may be used as shift-register elements or as type 'T' flip-flops for counter and toggle applications. The MC14013B CMOS SSI is a low-power complimentary MOS.

This device contains protection circuitry to guard against damage due to high static voltage or electric fields. However, precautions must be taken to avoid applications of any voltage higher than the maximum rated voltages to this high-impedance circuit. For proper operation, Vin and Vout should be constrained to the range Vss [much-smaller-than] (Vin or Vout) [much-smaller-than] Gnd. Unused inputs must always be tied to an appropriate logic voltage level (e.g. either Vcc or Gnd). Unused outputs must be left open.

The circuit, as described above, is a so-called "Radio Controlled Electronic Switch". It can be used to switch on/off anything electrical, whatever it is. Here are a couple of examples : navigation lights, landing gear, sound systems, glow plug driver, bomb release, parachute, search lights, gyros, and so on.

Fig. A shows the regular setup for common accessories such as motors, glow plugs, bomb-doors, relays, etc. If you like to hook it up to a camera, see Fig. B. Note that for the Camera Shutter version the value for R1 and R2 is different (100K). Please note that this system will not work with PCM.

Heart of the circuit is a CMOS Dual 'D' Flip-Flop MC14013B. The input Flip-Flop is designed as a monostable pulse generator by means of R1, P1 and C1 connected between 'Q' and the RESET input, which produces a preset pulse-length set by the adjustable potentiometer and starts at the rising edge of the input pulse. When this monostable times out it's inverted 'Q' signal goes high and clocks the output stage of the Flip-Flop, which is used as a normal type 'D', to sample the input pulse. If the duration of the input pulse is longer than the preset monostable pulse, then a logic high level will be clocked to the output of the 'D' type. A shorter input pulse will cause a logic low to be clocked to the output. In short, both halves of the IC perform two different logic functions.

The output drives the output device which in this circuit is the IRFZ42 TMOS FET. It needs only 2-volt on it's gate to fully turn-on and has an rDS-ON resistance of only 0.028 ohm. To invert the operation of the r/c switch, you can connect R2 either to pin 12 or 13 of the MC14013B. This circuit is easy in design and to built and can easily be done using vector board, vero-board, or whatever. The complete unit measures 5/8" by 1-1/4" but it can be a lot smaller by choosing SMT components, probably 3/8" x 1/2". You can use a case or heat-shrink. This unit is NOT meant as a motor-switch for electric flight.

Adjusting the Switch
To test the unit hookup a light and a battery, making sure the + of the battery goes to the Drain of Q1. Adjust the potentiometer P1 to somewhere in the middle and set the transmitter function of your choice (say the throttle) to the point where you wish to switch the unit. Now adjust the potentiometer P1 to the point the light comes on. If it does, your unit functions properly and you can play with whatever other setup you have in mind. If you intend to use this unit as a on-board glow driver, make sure to use heavy wiring between glow plug, battery and r/s switch. A 'Y'-lead to the throttle servo is required. If you use this unit to switch relays or a small dc-motor, then a 'spark eliminator' diode (1N4001) is required. Cathode of the diode goes to the '+' side of the battery. (See diagram).
At the top you see the Surface Mount version (SMT), measuring 15mm x 17mm!

The IRFZ42 is a TMOS Power FET and can be costly (approx. S$18). Other substitutes like the IRFZ44 will work too. Watch for static discharge with this one!

IC1, the MC14013B, is a CMOS SSI type Dual Flip-Flop. It features a direct pin-for-pin replacement with the CD4013B, NTE4013B, ECG4013B, and others.

The output signal can be inverted by selecting either pin 12 or 13. To make life easier, you could install a miniature on-on switch.

The middle contact going to R2, and pin-12 & 13 to the other two contacts.

1 comment:

Jorge Boullosa said...

Good blog good Article! I was find similar circuit for launch rocket missile from RC Airplane :)
My blog is
Only spanish :S
Regards! Jorge B