Altimeter Circuit

Posted on Dec 21, 2012

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 with 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 thermal offset drift; resistors R5 R13 R14 R22 MUST BE METAL 1%.

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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 throttle, 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 disposition 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. 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...

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