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LCD RPM display

Tacho measure tacho blank
baterry hollder angle view
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Below there is a picture of tachometer made by polish RC modeler - high workmanship (according to me ;-) )
click on picture to englare


Generally tachometer is a gauge to measure rotation speed of any rotating things. In RC hobby tachometer is the instrument for mesauring and monitoring propeller revolutions per minute (RPM). This tachometer has some additional functionality like: thrust, pitch speed and power estimating. I would like to emphasize word "estimating", because accurate computation above mentioned values is a complex function of many factors, so there are not any simple methods for precise calculation.

Technical Specifications - Characteristics

schemat multimetru

Tachometer use photoresistor as a photo sensor to detect light pulses. Those pusles are amplified in U2 two stage amplifier and then counted and converted into RPM number by ATMega8L µC. More specifically, µC count how many times per second, photo element “see” the blade of propeller and then use that information along with the number of blades of your propeller to determine the propeller’s RPM and display it on LCD.  

Part List:
Element Value/Type Case Remarks
R1 51k 1206  
R2 51k 1206  
R3 51k 1206
R4 51k 1206  
R5 51k 1206  
R6 51k 1206  
R7 51k 1206  
R8 51k 1206  
R9 51k 1206  
R10 15k 1206 10-30k are allowed
R11 1M 1206   
R12 1M 1206  
R13      Photoresistor
C1 2u2 1206    
C2 100n 1206    
C3 100n 1206   
C4 100n 1206  
C5 100n 1206  
C6 33p 1206  
C7 33p 1206    
C8 10u SMD C  
L1 47u 1210   
Y1 4MHz SMD  
BT1 Battery Holder     
U2 LM358 SO-8  
LPH7366 LCD LPH7366  LCD used in NOKIA 5110 phones
S1 SWITCH     
S1 SWITCH     

Layout - top layer Layout - bottom layer
click on picture to englare click on picture to englare

Picture of PCB is here. There are two versions of PCB - normal and mirrored. I think, that anyone who makes PCB's will know, which one schould be used to produce right PCB.

To provide properly working conditions, LCD pads on PCB should be perfectly clean. Clean them directly before LCD mounting using ethanol, propanol or any other chemical.


Because µC is in TQFP package, program it after soldering  all the elements on PCB. Programming signals are delivered through pads on PCB. You  can
make the programming cable useing old PC 5.25''FDD connector or simply solder wires to pads only for programming time.
Remember - some programmers source connected electronics with +5V. For safety mount LCD after programmer disconnect.

After connecting µC to programmer, check if µC is "visible" for programmer. When everything is fine:

HEXes included in ZIP file differ from mine tachometer code version. I made some improvements in source code, but I did not check it. If You notice any problems - mail to me.


Switch on tachometer by pressing any of switches. To perform propeller RPM measurement, set tachometer photo sensor near rotating propeller. When You use artificial light, set tachometer photo sensor in place, where propeller blades cross light beam. Only DC sourced artificial lights are proper for measuring conditions. AC sourced lights are blinking, even if it looks stable.
If You intend to use thrust, pitch speed and power estimating, You have to enter propeller characteristics. Only basic characteristics are needed:
By the way, number of propeller blades is necessary for proper RPM calculation.

To enter mentioned above information, press and hold both switches until first adjusted parameter appear. Enter propeller diameter by pressing left or right switch to decrease or increase value in range 1-99 [inch]. Release both switches for c.a. 5 seconds. The same way set propeller pitch and number of blades.

To save energy, tachometer automaticly switch off itself within time c.a. 15-30 second, depending on measure state.

Disclaimer and Copyright: Read on polish site

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