Undercarriage - Murrells Models

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Undercarriage Servo Travel Limiter v.2

The design was to limit the travel of the nose wheel servo on a model helicopter, because the rear undercarriage was driven from the same channel of the TX the adjustment could not be done by the TX, the nose wheel servo was enclosed in the fusealage and could not be accessed for easy adjustment.

The circuit uses a PIC 16F628 running at 4Mhz with very few external components. It uses an input pin Port B0 to detect the pulse from the receiver and then limit the pulse before re-generating the pulse on pin Port B3.
Of the DIL switches, only Port B7 & B4 are used.
See Picture of the switches, no.1 is at the top
1. Programming mode
4. Servo Reverse

Setup Instructions
Set pots to middle positions, set switch 1 on, power up, using an insulated screwdriver adjust the left pot for low position and right pot for high position, when happy with servo travel move switch 1 off. Apply servo reversing via switch 4 if required.
Suggested using some hot melt glue over switches to stop them moving due to vibration (pots are only active when switch 1 is on so any movement to their settings will be ignored)

Circuit Features
The +ve voltage is limited to 5.1v by the zenner diode just incase more than 5.5v was available from the RX batteries.
The pots are suseptable to stray capacitance, if you try adjusting them using an non-insulated screwdriver the settings will change when you remove the screwdriver from the pot.

Parts List:



47k ohms






Resonator 4Mhz


18 ohms


270 ohms


Zenner Diode 5.1v 1.3w


DIL switch 4 way

If you wish to purchase pre-programmed PIC's then get in touch, see email address on 'For Sale' pages.

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