rc switch

Simple RC on/off SW


  1. rcsw.zip


Please note that a PCB is now available for this project which is not shown in the drawings, these still show the stripboard layout.


The switch unit to be described in this article can be used for switching on and off any electrical system you wish, it could be an onboard glow plug driver, landing lights, airborne sound system or even a rocket motor to assist take off. It was inspired by a request from someone on the internet looking for circuits for an RC switch unit, the responses that he got fell into two basic designs.

The first design type used a system where the output pulse from the receiver was converted into a voltage, and this voltage was then compared to a reference voltage, this is a very simple technique which only needs one IC to implement it, but has a draw back in that it suffers from a large amount of hysteresis, this is the difference between the switch on point and the switch off point.

The second design type used a technique to compare the receiver output pulse width, with a reference pulse. This did not suffer from the hysteresis problem, but most of the designs used two different digital logic ICs but only used half of each one, resulting in a larger more bulky unit

It was decided to see if a design could be developed which used the second technique, but used only one IC, the result is the design presented here in this article.

The Technical bit

The circuit is based around a CMOS digital logic IC, which is a 4013 dual 'D' type flip flop. The first half of this IC is made to operate as a monostable pulse generator by the action of R1, RV1 and C1 connected between the 'Q' output and the reset input. This monostable produces a pulse of preset length set by RV1 which starts with the rising edge of the input pulse. When this monostable times out it's inverted 'Q' output will go high, this transition will clock the second half of IC1, which is used as a normal 'D' type flip flop, this will sample the input pulse. If the input pulse is of longer duration than the preset monostable pulse, then a logic high level will be clocked to the output of the 'D' type. But if the input pulse is shorter than the preset pulse, then a logic low will be clocked to the output. With this configuration we are using both halves of this IC to perform two different logic functions.

The output 'D' type flip flop is used to drive the output device which is a logic level drive MOS-FET, this device only needs 4.5v on it's gate to be fully turned on, and have an ON resistance of only 0.055 of an ohm. A side effect of this technique is that depending which output of the second flip flop we use, we can invert the operation of the switch, this is accomplished by connecting R3 to either pin 13 or pin 12 of IC1.


The unit is very simple in design and so is fairly easy to build. Cut the veroboard to size first and check it is a snug fit in the case. Then cut the copper tracks in the positions shown in the drawing of the track side of the board. Now you can fit the components making sure to fit the two wire links that are under IC1, and shown in the component layout as dotted lines. Also make sure that you link pins 6 and 7 on the trackside of the board. Once completed the board should be fitted into the case which has been modified as per the drawing.

Setting and Use

Once the unit is connected to the system that you want to switch, set the pot RV1 to its mid position and then set the transmitter function to the point you want the switching to take place, then adjust the pot RV1 so that the unit switches on. If used to switch something like a glow plug then the plug can be switched directly. But if you want to switch something like a relay or motor then you will need to fit a diode (1N4001) across the relay or motor as shown in the circuit diagrams. If you are going to use this unit as a glow driver then it will need to be connected to the throttle servo with a 'Y' lead, also make sure that you use fairly heavy wire to connect the plug and the ni-cad to the switch unit.

This unit is not meant to be used as a motor switch for electric flight, but if the output stage was beefed up a bit with some extra FET's it would make a reasonable starting point.