Simple timers
a go - go
Andrew H's flight timer
Circuit taken from articles by Robert Pearce (EFUK issue 39) and Rainer Krafft (Model World – April 1997)
There are two parts to the timing system.
A) The Timer. (Seen above)
The timer is made from a capacitor, a resistor and a transistor. The voltage of the capacitor is used to switch on the transistor which controls the flight motor.
We charge up the capacitor and then it drains through the resistor. As it runs down the voltage in the capacitor drops.
The transistor is fully on above 1.3v and fully off below 0.8v. From 1.3v to 0.8v gradually switches the flight motor off. By changing resistor we can select the speed the capacitor runs down. If we charge the capacitor up to higher voltages the flight motor will be fully on for longer. An example of this is way down at the bottom of the page ......... Mark's version of the timer
The flight motor does not stop suddenly causing the plane to stall. We can select the resistor and voltage to suit slow climbing indoor planes or fast climbing outdoor planes.
The timer is carried inside the plane. Wires from the capacitor are connected to the outside of the plane so we can charge the capacitor. The capacitor is charged using a hand help circuit which is adjustable so we can vary the voltage we want to charge the capacitor upto. However with experience you might find you can't be bothered changing the voltage of the cap and just let the plane run down a bit before launching it.
Capacitor Charger .
The voltage of the PP3 battery is varied from 0 to 9 volts using a variable resistor (Poteniometer or "Pot".) Touch the green and black charger contacts onto the green and black timer contacts for one second. The pot can be either a single turn rotary knob type or a mulitiple turn screw drive affair - makes no difference. Use a 10 k Ohm one, but a larger one would work, just makes it more sensitive.
Disconnect the PP3 battery when not in use.
Here are a couple of Andrew's big f/f electric planes. Made of Blue foam and Depron
Mark's simple test plane timer is bottom right
Below, simple (crude !) capacitor charger. PP3 battery is fixed to the bit of wood. The output of the Pot is marked so you dont have to use a mulit meter to select the voltage at the flying site. Down is short flight, up is long flight, simple enough (if the planes flies .... )
Above, Mark's multi turn pot allows more accurate adjustment and is glued onto the PP3 battery plug thingy. The wires need to be a bit longer really. Voltage is set at the beginning of the evening with a multi meter. Maybe use different pots for different planes?
Mark's timer is the same deign as Andrew's but uses much smaller parts so it is suitable for sub 10g indoor planes. The parts have been selected to fit together easily, this limits the choice of parts available. However, the one now in regular use uses a SOT 23 size transistor and 805 size capacitor and resistor, which fit nicely between the pins on the SOT 23 transistor. The transistor can handle 400mA which should be plenty for a tiny F/F plane.
Yellow wire comes FROM the flight motor Black goes to the battery negative.
Left is the resistor, middle is the capacitor, right is the transistor
With the transistor's lettering facing up at you, the negative if the pin on the right, as shown here. This may vary on other transistors
The transistor is fully on above 1.3v and fully off below 0.8v. So if you charge the capacitor to 1.5v the plane's motor produces full power. The capacitor will slowly discharges through the resistor. When it gets down to 1.3v the transistor starts slowly switching off the current to the flight motor until at 0.8v it is completely off and the motor is stopped.
So, if we charge the capacitor up to say, 3v the full power motor run will be much longer, but the slow throttle back from 1.3v to 0.8v will be the same.
So, if we use a huge resistor the time taken to go from 1.3v to 0.8v will be longer. We will get a longer throttle back period. But we muct charge the capacitor up less otherwise it will take ages to get down to 1.3v
Trial and error shows a 22 Million Ohm resistor and a 4.7 miro Farad capacitor (47uF) give a throttle back period of about 40 seconds. If we charge the capacitor up to 1.5v the time from 1.5 to 1.3v (the full throttle period) is about 15 seconds.
To achieve acurate capaciotr voltages Mark uses a 22 turn potentiometer which is much more accurate and can be adjusted with a screw driver so it don't accidentrly get bashed and re-adjust the voltage resulting in a trip to Belguim to get the plane back.
Due to the small voltages now used the system can be a bit sensitive when the motor is less than half power, most F/F planes would power gliding by then so it is not thought to be too mcuh hassle given the cost, weight and ease of use of the timer .... so wadya want for practickly nothing !!???
Tips !
If you want to use more power you could use a more power full transistor, like Andrew does. The added weight would not be a problem on a bigger plane.
It does not make any difference if the capacitor is soldered to the transitor before the resistor or after it. It is easier to solder the resistor to the cap, then hold the transistor with tweezers and solder the transistor onto the cap/resistor. Add the wires last.
If you wanna switch the motor off - short the capacitor by touching a metal object from one end to the other.
If you aways use a yellow wire from the motor it is Aeronutz standard, same as the remote control planes.
Buying the parts to make a micro timer can be a hassle as some vendors have minium order quantities. So Mark can sell you the parts for a quid or so. Email for a confusing reply, profits to Aeronutz.
These timers could be used for all sorts of purposes ........ boiling eggs, eh?
See also our web page on electric free flight, voltage switch and variable voltage circuit which are relevant and helpfull.
