I proposed the Rocket Payload project to my local radio control club members. The feedback I got was, "Well, that's kinda nice, but can you make something to help locate our lost rockets." So this project was born.

The commercial products (yes, they exist) are either too big/heavy and therefore relatively cheap and dumb, or small/light/smart and unaffordable. In the first category are the beeper finders: some don't even have an on/off switch -- they just start screaming at 120dB when you plug the battery into it. My experience with using sound for location (in Search and Rescue and my quad copters) is that it doesn't go very far. The really expensive stuff (> $250) uses a GPS module and some kind of telemetry based system to transmit coordinates back to a base station or handset.

My first attempt at a Rocket Finder was to couple a beeper (good for when you are less than 20m away from the target) with a cheap/small ELRS receiver that can transmit a simple RSSI (Received Signal Strength Indicator) value to a radio control transmitter module that nearly everyone in the club has. That worked, but ELRS can transmit GPS data along with the other telemetry data which usually requires a flight controller. The addition of a flight controller would increase the cost and size of the Finder past its required parameters.

I asked my buddy Claude (who works for Anthropic) if he could help me add a small/inexpensive GPS module as an option to the original Rocket Finder and he gladly volunteered for the job.

System Diagram:

There are only 3 or 4 components: The Finder, the ELRS receiver, the optional GPS module and a battery for power.

There is a hall effect switch on the Finder that arms it when a magnet is place nearby. It then supplies power to the receiver and GPS. It also emits a short beep every five seconds to confirm that it is armed and also help in locating it when the user is close enough to hear it. The GPS uses a UART to transmit its data in UMEA format, at 9600 baud, to the Finder, which decodes it and re transmits the GPS data, in ELRS or CRSF format, to the ELRS receiver that transmits it over its telemetry link. The GPS is configured to only send one message at a 5Hz rate. The message information includes Lat/Long, Altitude and number of satellites. 

The user gets a display of the GPS coordinates and altitude on his handset. It might be possible to record the maximum altitude of the rocket. After the flight the user can use the GPS coordinates to lead them to the rocket/finder which is still beeping every 5 seconds. Once the user recovers the rocket the Finder can be disarmed by placing the magnet near the Finder, which gives a confirming set of beeps and goes to sleep with very low current drain.

Specifications:

• Size: 11 x 20 x 8-10 mm. Might fit in a 1/2" tube.
• Weight: < 10g with battery.
• Battery: 100mAh 502512 LiPo, 5 x 25 x 12 mm, 2.5g
• Operating supply voltage range: 4.2V - 3.0V.
• Standby current drain: < 10uA.
• Operating current drain: 50mA w/o GPS, 80mA with GPS.
• Range: TBD, depending upon ELRS receiver, but expecting 300-500m minimum.
• Cost: It depends upon which options selected, but the basic Rocket Finder should be less than $5, including battery.

Schematic:

The micro-controller is a PIC16F15214 in a UDF8 package. It is internally clocked at 32MHz. The PIC has only one MSSP serial communication module, which is used to talk to the receiver at 430 kbaud. The UART signal to the GPS is much slower at 9600 baud so it is big-banged. The PIC is inexpensive -- about $0.5/each in low quantity. Programming the PIC is accomplished via the 6-pin JST-SH connector, which also is used to connect the ELRS receiver and GPS module to the Finder.

The hall-effect switch has hysteresis for debouncing and draws little current. And it's cheap, or at least it was in the time of no tariffs.

The FDC6330 load switch provides battery power to the receiver and GPS. Note that there is only 1 resistor on this schematic --...