DIY Wireless Timer

Basic single lane Radio Timer

From (contact Gao)  now at
1 x Single channel Wireless Garage door controller
2 x CB-1 long range transmitters

from electronic parts suppliers
AA battery holders – 2 x 9volt and 1 x 12volt
2 x 9 volt batteries
20 x AA batteries
3 x Switches
2 x Transistors
2 x Resistors
2 x Capacitors
RJ11 plugs and sockets wires
4 strand or 2 strand telephone lead.
1 x BTS Timer Which you can order directly from BTS Here
or if you must, a DIY modified Electronic stopwatch (this is very tricky for most!)

2 x Tape switches – Greg Ambler pattern – see How to build a Tape Switch here……

Step 1 get a BTS Timer, and you can proceed directly to Step 2!
But if you are too cheap or too poor, or have too much fear of anything imported to your fair land, to get a BTS timer….read on.

Open up your imported stop watch, and locate the stop start switch.
Strip back two of the wires in a short length of 4 strand telephone lead.
Solder the wires to each contact of the stop/start switch.
On the other end of the lead, fit a rj12 connector so that the two wires soldered to the stop watch switch are the centre two leads in the rj12 connector.
(BTS Tape switches and timer also use the centre two wires in the rj12 connectors)
File a little of the watch case so that the telephone cable can pass through the case, but also be firmly held (clamped) in place when the case is shut.

Re assemble the Stopwatch.
(BTS Timer will save you all of that effort, and it works perfectly with this system)

Step 2 – mount the receiver

Mount the wireless receiver in a box with an 8 x AA battery holder (12 volt s), and on/off switch.
Set the transmitter to momentary operation.
Connect a telephone cable to the relay contacts, and on the other end, using an rj 12 connector, and a female to female rj12 adaptor, join that to the stopwatch cable.
Use one of the transmitters to check that the receiver works, and the stopwatch starts and stops each time you press the transmitter button.

Step 3 – make a delay circuit
Because the front wheel of the skateboard, crossing the tape switch will start the timer, and the back wheels will then stop the timer, we need to delay the closed state of the tape switch long enough for the rear wheels to cross the tape switch, so that the timer runs until the front wheels cross the tape switch at the finish of the course. (and also so that the transmitter stays on long enough to trigger the receiver.) This only needs a delay of about 2 seconds.

Using a transistor as a simple “delay” switch.
The delay circuit consists of a 9volt battery, a capacitor, a transistor, and a resistor.
A transistor has three terminals, being the Collector, the Emitter, and the Base. When the base of a transistor is charged, it transforms the connection between Collector and Emitter from an open switch (non-conductor) to a closed switch (conductor). By using a capacitor and a resistor you can keep the base of the transistor charged for a period of time, after the power is disconnected from the circuit. (This is a very basic summary of the use of a transistor as a switch,and ignors the full capabilities of transistors in general but it is all that we need for this application)

Assemble delay circuit

The positive terminal of a 9 volt battery is connected to the capacitor via the tape-switch, The positive terminal of the capacitor is then connected to the base of the transistor via a resistor. When the tape switch is crossed, the battery fills the capacitor instantly, which is slowly discharged via the resistor, into the base of the transistor. The negative terminals of the capacitor and battery are connected to the Collector of the transistor, ie circuit ground. Next join a lead to the emitter of the transistor, and another lead to the collector of the transmitter. These two leads run to the radio transmitter switch.
Exact size and value of resistor and capacitor is determined by the delay that you need, and you can find out suitable values from most electronics books. (failed compact flouro lights, is a good source of free transistors!)

Step 4 Modify the transmitter.

Open the transmitter case, and locate the activation switch on the circuit board. Solder the two leads from the delay circuit to the contacts from the switch. (I used rj12 socket and plug to join the delay circuit to the on switch on the transmitter) Connect the tape switch to the delay circuit and test. The transmitter should stay on for a few seconds.
Next connect the 6 x aa (9volt) battery holder, via some leads and a switch to the battery contacts in the transmitter.

Step 5 Assemble transmitter boxes.

Put the delay circuit and its battery into the box, with a rj12 female socket set into the side of the case. This socket connects the tape-switch to the battery and the capacitor in the delay circuit, using centre two connectors.
Hold components in place with double sided tape.
Connect the delay circuit leads to the transmitter, and put the transmitter and its battery into the case. Then test and close the case.
(note that the transmitter shown is not the CB-1 transmitter, and is using a single 9volt battery for the delay circuit, and another 9 volt battery, mounted behind the transmitter for transmitter power)

Step 6 – set it up and use it

Connect the tape switches, turn everything on, and start racing.

AA batteries give much better duration than the 9volt batteries for the transmitter, and should last about 20 or 30 sessions (guessing, have not run flat yet!).

This setup is good for about 120 meters in a ditch, and about 200 meters in open ground.
The best part is the portability and simplicity of use, with no cables to trip over.

Cost should be under $100 for the wireless link.

I used a dual channel controller, for this set-up but unfortunately it does not work for twin lane racing, as the second channel to trigger has to wait until the first channel has stopped receiving. For twin land racing it is best to use two single channel receivers and 4 transmitters.


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