A very simple FET timer for brushed 6/7mm pager motors
The following notes describe the
construction of a very simple electronic timer for indoor free flight made from
3 major components, a FET (Field Effect Transistor), a resistor and a
capacitor. This thread guides the interested, hesitant newcomer through the
culture shock of an electronic discipline which many find tricky to get to
terms with. …..worry ye not, this is not as difficult as you think!!
This timer has been developed extensively by Peterborough MFC over 3 years and
can be made by anyone with average soldering skills. It is intended for use
with a single cell LiPo giving motor run times
anywhere in the range 5-55 secs. The timer has been
tested with a stalled motor to 4amps, weighs 0.8gm less wires, costs less than
£1 and takes 15mins to make.
The theory (you do not need to read this but it will help no end if you do make
the effort)
If you apply a voltage to the Gate (G) of a FET, the Source/Drain (S/D) pins
will conduct. If you put a battery and motor in this circuit
, it will switch on. You get the voltage on (G) by charging or ‘zapping’
the capacitor with an external battery. The resistor gradually drains the
capacitor until the (G) voltage dissipates. The (S/D) resistance increases and
the motor switches off. By varying the magnitude of
the voltage applied to the capacitor, you can vary the motor run time.
Making the FET timer
Refer to the MK1_timer graphic. Solder the capacitor and
resistor to the (G) and (S) pins of the FET. The capacitor is polarised and must go brown stripe (or positive) to (G).
The resistor doesn’t care which way around it goes. Solder a red wire to the
(D) pin (on the back of the FET) and a black wire to the(S) pin. A 10-15 Watt
iron with a 1mm tip makes things easy. Maplins are a
good source but check the tip before you buy and treat yourself to the finest
tip you can find.
MK1 timer
graphic.
All the bits you will need
to assemble a FET timer
Wiring it up into a motor/battery circuit
Refer to the ‘FET_motor_hookup’ graphic.
Access the ‘connectors’ thread on FlyQuiet (http://www.flyquiet.co.uk/smf/index.php?topic=1332.0)
and use a non polarised type 4 set up on the motor
and polarised type 6 on the battery. On the first set
up, the wise man builds a test rig to become familiar with the use and
operation of the timer.
FET_motor_hookup……….how to
connect motor and battery
Making a Zapper
To vary the run time you have to have a system that varies the voltage applied
to the capacitor. The PMFC use a 9v PP3 and a 4k7 wire
wound linear pot to ‘potential divide’ the battery voltage, to say, 3.5 - 9v. A
schematic circuit is shown as zapper_scem. Very easy to make as all the bits are chunky. My Zapper is
shown and is very tatty having survived 3 seasons. It was made from scrap stuff
so I do not have part numbers to give you. Maplins or
Rapid will provide.
Zapper_scem : Basically a
potential dividing circuit to provide 4-9volt to the FET pins
Typical models
The timer has seen service in 15 or so models, samples of which are shown in
the indoor_stuff graphic. These are 2mm Depron with tissue printed finish applied with Johnsons Klear running 6/7mm
coreless pagers with 50/90/130mah 1s LiPo and PMFC
FET timers……AUW ranges from 12.4 to 19.5gm. The models give about 10 30 sec
motor runs before recharging ………a Didel BiCha is the PMFC preferred charger, capable of charge
rates of 10-150 ma in 10 ma steps.
Indoor_stuff: Some
examples of what can be run with a FET timer, a 6/7mm pager motor a 1s
50-120mah LiPo
and a sheet of 2mm Depron
If you doubt your ability to specify motor/battery/prop then tell me what you
want to do and I can advise accordingly for both indoor and light outdoor FF
stuff.
I hope these notes may help in encouraging those wanting to try electric FF to
chance their arm and get stuck in as I and the Peterborough MFC will be all too
willing to help out if you get stuck.
The Mk2 FET timer
This is a modification of the basic
Peterborough timer and replaces the fixed 470k resistor with a variable
potentiometer. Part number 67-0656 (Rapid) gives you a 1M side mount cermet pot which will provide motor run times of 5s to
130s. The disadvantage of this scheme is that it is trickier to set but you do
not have to make a Zapper. All you need is a 9v pp3 battery. For those who
cannot contemplate soldering, Ted Szklaruk provides a
ready built timer at a small cost. Refer to the ‘Contacts’ section and contact
Ted through the Chairman Bernie Nichols.
The Mk3 FET timer
A further variation of the FET timer is
again to negate the need to make a Zapper. If you make the resistor ‘pluggable’
you can vary the motor run time using the resistor value as the variable.
Typical motor run times for each resistor are as follows……….
Resistor value Rapid part no Approx motor run time
(sec)
150k 64-0052 19
270k 64-1012 34
390k 64-1014 49
470k 64-0142 59
560k 64-1016 71
680k 64-0068 86
820k 64-1018 104
1000k 64-0072 127
All you need to start the motor is a 9v
pp3 battery.
Return to
Technical Articles page.
A Peterborough Timer and Zapper from Dave Andreski , US
In April 2011, I received an email from
Dave Andreski who had made a Peterborough FET timer
and Zapper from notes in the Peterborough website ‘Technical Articles/A Simple
FET Timer for brushed 7/8mm pager motors’.
Dave did not have much trouble in
finding the necessary components in the US and used the recommended STD12NF06L
FET. A bare timer came out at 0.64gm and 0.94gm with wires. He included some
photographs of his endeavours, together with an
outdoor FF model, The Truant, weighing in at 33 gm ready to fly. He made a very
nice job of his Zapper. I must rebuild mine someday. He did not say what motor
he was using but it might be a DD 12mm
can motor.
Good to know that writing projects up
is of use to other modellers.
Below is another FET timer build , this
time from flyced of Quebec, Canada.
