We were recently asked by a client if it was possible to externally trigger our Doongara Balloon Cut-Down device with a simple RC Switch. The external trigger was originally designed to allow the Doongara to be connected to an Iridium modem to allow real-time remote control. While the Iridium option is great, it is also expensive. The option outlined below is intended for the cases where equipment costs outweigh the downsides of needing to maintain an RC link to the balloon.
Possible to RC Trigger Balloon Cut-Down?
Yes, it is possible to use an RC switch and a Doongara to RC trigger balloon cut-down. The information below details on how to connect a Doongara to an inexpensive (~$12) Turnigy RC Switch. This is not the only RC switch available, nor perhaps the best switch for this job. Rather this was the RC switch we had on-hand.
About Doongara’s External Trigger
The external trigger input on Doongara is configured so that an external open-collector circuit can generate the necessary signal. This allows either a direct connection to a microprocessor (i.e. Iridium Modem) or transistor (i.e. RC Switch). Doongara has a 4.7K resistor that pulls the trigger input high to 3.3V. The external trigger on Doongara, if programmed to be enabled, will activate when this trigger input is pulled low (to ground) for >500ms.
About Turnigy’s RC Switch
This is a simple RC switch that has a small microprocessor that decodes a PWM input and controls an output. A standard 3-wire servo connector provides the signal and power for this circuit. Two short pig-tail wires allow the device to be connected to the load to be switched. There are two jumpers to set the default power-on settings.
This device utilizes an open-drain circuit that can switch up to 10A at 30V. The final output control is comprised of two P-channel MOSFETs (TPC8107) that are connected source-to-source so that the load’s connection polarity does not matter.
The Problem for Doongara
Doongara’s microprocessor requires that the trigger voltage be less than 30% of its working voltage of 3.3V. Thus to trigger Doongara, the external trigger line must be held below 1V for 500ms. These P-channel MOSFETs have a gate threshold voltage of 1.6V on the unit I was working with (0.8 to 2V on the spec sheet). This means that when Doongara’s trigger is connected to the two output lines on the Turnigy’s RC Switch and it is activated, the line is only pulled down to ~1.6V. This does not cause the Doongara to trigger.
How to wire: RC Trigger Balloon Cut-Down
Luckily, a smaller SOT-23 N-channel MOSFET (or possibly a cheaper NPN transistor) drives the two P-Channel MOSFETs on this RC switch. It is possible to connect Doongara’s external trigger directly to this chip and have the RC switch provide the necessary external trigger signal.
To do this:
- First remove the heat shrink covering from the RC switch
- Then solder the black ground wire from Doongara to the brown PWM ground wire
- Then solder the yellow external trigger wire to the small SOT-23 chip on the side that only has one pin
- Finally wrap the RC switch back up in electrical tape to provide some strain relief for the wires