Gadget Freak

How high can you go? That's a question that model rocket enthusiast Jerry Baumeister wanted an answer to. Cheap enough to be used in even "risky" launches, Jerry's battery-powered device features a circuit designed around a Flash microcontroller that performs an analog-to-digital conversion of the voltage produced by an absolute pressure transducer. Altitude and operational data is then output to two LEDs. Since the output is in A/D units, Jerry just has to do a quick mathematical conversion and he's flying high. Or at least he knows if his rockets are.

Model Rocket Altimeter

This design originally appeared on Design News.

The following circuit was designed to provide an inexpensive, simple and easy to construct altitude recorder for small model and amateur rockets. Every effort was made to keep the design simple and within the capabilities of most amateur experimenters as well as to provide accurate and useful functionality. The circuit is small measuring 2 5/8 inches by 9/16 inches and weighs only about 0.3 ounces. It features PCB construction using standard size through-the-hole components. Also, it is cheap enough to be used in “risky” launches and durable enough to survive the occasional mishap.

Theory of operation:

The circuit is designed around a PIC 12F675 flash microcontroller (IC2). Pin 7 of IC2 performs an analog to digital conversion (A/D) of the voltage produced by T1 an absolute pressure transducer. The transducer used has a high level output (0.2 to 4.8 volt), which interfaces directly to the microcontroller. Pins 3 and 6 are used to output the altitude and operational data to LEDs. Switch S1 is used to trigger the output of the maximum-recorded altitude since “power-on”. Power is provided by a 23A “lighter” battery and IC1, a linear voltage regulator.

The microcontroller program is written in Microchip assembler using their MPLAB-IDE development software and programmed on a microchip Flash Starter Kit. The program is composed of a series of subroutines, which should make the program easy to modify if the builder so desires. A free copy of the program can be downloaded from jbgizmo.com or a preprogrammed microcontroller can be purchased from the same source.

The output of the circuit is in A/D units (0-1024) and must be converted to an altitude measurement mathematically. This mathematical calculation is described in detail in the operational portion of this document. An Excel spreadsheet chart is available for download from jbgizmo.com, which makes computing the altitude easy and it is more suitable for field use.

The Pressure transducer output equation:
Vout=Vs(0.009P-0.095)+- error
P=pressure in kPa
Vs=5 volt
Temp= 0-85 C

The altitude calculation:

Altitude in feet={[10log(ph/p)/5.2558797]-1}/ (-6,875,585.6)
p= sea level pressure (obtained from the local weather station or NOAA weather broadcast)
ph= station pressure measured and read from the circuit. These A/D units will need to be converted to inHg using this formula… station pressure inHg= [(A/D units * .032047)+3.11754]

Construction:

Construction is straightforward. Etch and drill a circuit board according to the pattern provided and solder the parts on the circuit board as indicated in the drawing. The only tricky procedure is the construction of the battery holder.

The battery holder is composed of the spring ends of two #1 safety pins. Cut and remove the clasp end of the safety pin, extend the ends through the PCB, fold ¼ inch over and solder flat against the foil. This makes a nice small battery holder but may be a little awkward to construct. If the builder would like, any “N” size battery holder can be used instead.

Operation

1. Install the battery with the positive (+) end towards the middle of the PCB. Secure the battery to the PCB with 3 layers of electricians tape. Stretch the tape tight when applying. If the PIC12F675 is socket mounted then it will be necessary to secure it to the PCB in the same manner. It is important that the battery and microcontroller chip are secure. Force a small paper-thin piece of plastic between the negative battery terminal and the negative battery "clip" to turn the circuit "off". Remove the plastic to turn the circuit "on".

2. Remove the plastic shim to turn the altimeter "on". The red LED will light in approximately 3 seconds and stay lit for 2 seconds. The green LED will then blink the left most digit of a 3 digit number, followed by a red LED blink. This sequence will repeat for the other two digits. The red LED will then start to blink once approximately every second indicating that it is actively reading and recording pressures. Write down the 3-digit number. This number is the current station pressure (preflight) in analog to digital converter units (A/D units). Record this number. The altimeter is ready for launch.

3. Load the altimeter into the payload compartment of the rocket.

4. Launch and recover the rocket.

5. Push the momentary switch (S1) for one second and release. This places the altimeter in "readout" mode.

6. The altimeter will blink the value of the lowest pressure that it recorded during the flight in A/D units. Record this number. The altimeter will continue to repeat this reading until reset.

7. If you know your launch site sea level pressure or altitude you can use the Altitude Conversion Chart to figure the maximum altitude obtained during flight.

8. If you want to do the math:
p= sea level pressure obtained from your local weather station or NOAA broadcast in inches of
Mercury
p1h= ground level reading
p2h= post flight reading
p1h= [(A/D pre-reading *0 .032047) +3.11754] +-error … this converts the A/D reading to in.Hg
p2h= [(A/D post-reading *0 .032047) +3.11754] +-error … this converts the A/D reading to in.Hg
error ranges between 0 and 1
preflight altitude at lunch site ={[10log(p1h/p)/5.2558797]-1}/ (-6,875,585.6)
post flight maximum altitude ={[10log(p2h/p)/5.2558797]-1}/ (-6,875,585.6)

9. If you don't want to do the math and close is good enough then multiply the pre flight A/D reading minus the post flight A/D reading by 32. (P1-P2)*32

10. To reset the altimeter for another flight turn it "off" by inserting the plastic shim between the negative battery clip and the battery and then turn it "on" by removing the plastic shim.