PRESSURE SENSOR BASED ALTIMETER CIRCUIT CONSTRUCTION AND TUNING NOTES 1) Introduction The circuit uses a absolute pressure sensor to measure barometric pressure, it depends on the altitude but not linearly, remember this using the circuit. The pressure sensor can be absolute or differential, this means that the output depends on absolute pressure or on the pressure difference between two ports of the device. It is a four terminal device: if a courrent flows between terminals 1 and 3 the DDP found between pins 2 and 4 is (almost) proportional to pressure (absolute or differential). The output depends from the current and from temperature: naturally this must compensated. The output is very small, then we need amplification. 2) What the circuit does The device has been built for towed gliders contests, in those contests gliders are towed by a big rc model to a fixed altitude and relased, after the relase the flight time is measured, and so on. To do this is required a device which measures the altitude and signals when the release altitude has been reached, the release is left up to glider's pilot. The circuit measures the air pressure and compare it whith a given reference (trigger altitude) when the pressures drops under the reference the command pulses sent from the receiver to the trottle servo are cutted off and replaced by a locally generated pulses which trottles the engine to a preset speed (usually a fast idle); when the altitude drops under the reference, the trottle control is switched back to the receiver. 3) The circuit The circuit is powered by the receiver's pack, the current drain is low, specially if compared to drain of servos. U1a VZ2 and Q1 are the CURRENT source of the sensor VZ2 is temperature compensated, use the listed component except for Q1 there you can use any pnp silicon transistor, resistor must be metal oxide 2%(better) or 5%. R4 must be adjusted to obtain about 2 Volt between pin 3 and 1 of Pressure Sensor, do not replace it whith a trimmer, replace the resistor, the DDP is not important, its stability IS VERY IMPORTANT. PS1 is the pressure sensor, R12 (METAL!) is there to compensate its thermal drift (See later). U1b is the first amplifier, adjust R23 to compensate its termal offset drift; resistors R5 R13 R14 R22 MUST BE METAL 1%. U1c give more amplification and add a voltage proportional to the trigger altitude, to be set using P2. Use metal resistor also here, p2 must be a GOOD quality miniature 10 turn trimmer, do not use cheap part here. The gain can be adjusted replacing R19, the value given set the output to be 1mV = 1 meter if you want 1 mv = 1 foot you must set R19 =~ 60k. U1d is the comparator, it compares the output of U1c to a reference voltage provided by the group VZ1 R9 R10 R11 R16 R20. The four NOR of 4001 are the digital switch, it switch the pulse output (U3d pin 11) from receiver to the output of NE555; the specs are more relaxed here you can use standard part. P1 adjust the idle trottle, you can use a single turn trimmer, multiple turn trimmer can be used but requires a longer time to adjust on the field. CN1 is the servo in and out, it carries also the power for the altimeter. Get a servo extender (wire with a male conn. in one side and female in the other), split it in the middle, use the two half as input and output cable, obviously connect the wires according to the dispostion of your rx and servo. CN2 is the meter connector, adjust the voltage between pin 2 and 3 to be equal to the trigger altitude required DO THIS JUST BEFORE STARTING THE ENGINE. 4) Tuning and adjusting Before soldering it, measure the resistance between pins 1 and 3 of PS1, use a good multimeter, R12 must be about 1.5 times the measured resistance; the value printed on the circuit is only orientative. Double check all the circuit, ask a friend to repeat the check for you. Check also the quality of the work: soldering, isolations, fastening of part to pcb, etc. Power on, look for smoke....no smoke? OK go on. Check the current source VZ2 U1a etc. measure the voltage betveen pin 3 and 1 of PS1: it must be about 2 Volt (1.8 - 2.3 can be ok) if it near 0 or near power check all again, else adjust (replace or mount one resistor in parallel) R4 (higher R4 higher Voltage). Connect a millivoltmeter betveen pin 2 and 3 of cn2, adjust P2 to read 50-100mv, going up and down the stairs you must see the reading change, about 1mV each meter. First Thermal adjust: disconnect R13 from PS1 pin 4 connect it to PS1 pin 2, (short the input of OpAmp togeter) and start a thermal cicle. For thermal cicle i mean: put all but battery and millivoltmeter in the refrigerator, wait 15 minutes record the reading and, put all in free air wait ... record the readig, warm all to 50 Centigrades (please do you the conversion ...) (use a table light for this) and repeat. Modify R23 to minimize the drift, repeat all the cicle until you are satisfied by result (the quality of the result is proportional to your patience). In some cases r23 must be disconnected from pin 5 and connected to pin 6 of U1b. Reconnect R13 to PS1 pin 4, reconnect the millivolmeter and readjust P2, repat the thermal cycle, but more accurately (now you need a thermometer) plot the output reading versus themperature between 0 and 40 Centigrade. Adjust R12 for minimum drift: this means replace R12, repeat all and compare the plots if the second is better you are going in the correct direction etc. A good plot look like this BAD GOOD xxxx xxx xxxx xxxxx xxxxxx xxxx 0 20 40 C AND NOT 0 20 40 C If you obtain a good plot you are near the end. Calibration: use clear plastic tube, fold it in U shape, half fill it of water, connect one side of tube to the port of Press. Sensor;move the U to level the water betveen the two half of U tube, adjust p2 to read 0.0 Mv. Move the U to have a difference of water level of 14.5 inch: this correspond to about 1000 feet of altitude; correct R19 to obtain the correct reading a good choice can be 500.0 mV (1 mV = 2 feet). _____ _________ ___ | __| _______ > to PS1 port | | A |x| | | | |x| | | 14.5"|x| | | | |x| | | | |x| | |__ V |x| |x| |x| |x|______|x| |xxxxxxxxxx| ---------- You are ready. fix all, paint both sides of pcb whith a clear paint, let it dry. (The circuit is sensitive to humidity). Mount it in your model, adjust p2 to have a negative reading (this switch on the internal pulse generator), switch on Tx and Rx, verify the trottle: it must be controlled by P1, adjust p1 for a fast idle (fast..to be safe), adjust p2 to have a positive reading, the trottle is now under TX control. 5) Installation in the model Place the altimeter inside the model in accessible position, avoid aerodinamic pressure change (how to explain? close has many holes has you can). The altimeter get power from the receiver and then is switched on and off from the rx switch. 6) Use At field, before refueling, connect the millivoltmeter adjust P2 to required trigger altitude, test the operativity of trottle, disconnect millivoltmeter and go. If weather change repeat the adjustment, (barometric pressure change with weather). The altimeter is temperature sensitive (you learned it?) park the model in a fresh place, do not use dark colors and so on. Part List PS1 Pressure sensor Motorola MPX100AP VZ1,VZ2 Reference voltage diode Motorola LM285-2.5 D1 Diode Any 1N4148 or equiv. Q1 PNP Transistor Any BC307 BC308 BC320 .... U1 Quad Op Amp Motorola LM324 U2 Timer Any NE555 U3 Quad Cmos Nor Any 4001 P1 single turn miniature trimmer 15K P2 10 turns miniature trimmer 1.8K C1 Ceramic capacitor 10nf C2 Ceramic capacitor 120nf All resistor 1/8W metal 1% or 2%