RADIO CONTROL SWITCH UNIT
By Ken Hewitt

Originally published in the January 1996 issue of
Radio Control Models and Electronics


Introduction

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.


Construction

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.


Parts List

C1              22nF

R1              47k
R2              10k
R3              1k
RV1             100k

IC1             CD4013

Q1              BUK555-60B

Servo Lead	Available from local model shop	


Author

See web page under ordering parts.