Hobby Rev Counter Model ‘c’


The Hobby Rev Counter is a useful tool to improve the performance of your engine. It will accurately store the maximum RPM of your engine while in use. Installation is easy. The unit is small and can easily be fitted to your model. The unit will cater for different gear ratios. This is preset able using the mode switch.


Some modelers will install the unit permanently in the radio box. Others will connect it only to test the model during preparation. The unit is light thus if you want to use it only for testing then stick it to the deck of your boat or another suitable place near the engine using Velcro or similar. The unit is also splash proof thus it should not fail if it gets a little water. If you dunk it , it will get water and die. The best solution is to put it in a sealed plastic bag. Avoid heat on any part of the unit or sensor.

The unit has a built in battery however if you have problems with this you can power it up from an external battery. Connect a 3 or 4 cell battery to the Red + and Black - wires. The unit draws very little current thus any small battery will do. Tie the battery down with a cable tie.

The built in battery can be charged using an ordinary RC charger by applying approximately 7Volts to the unit on the black and red wires. A Special charger is available. The unit must be charged for at least 10 hours @ 20mA before use. With the special charger the mains frequency x 2 is fed to the unit and displayed while charging.

The sensor is a Haul Effect sensor. This means that it detects the presence of a magnetic field. Supplied with the unit is a small bush fitted with a magnet. This bush is fitted onto the drive shaft of your model. It has a magnet glued into it. Should you wish to fit the sensor in a different position you can get a spare magnet. It is important that the magnet is fitted with the same polarity as the bush. Ie, south pole to the outside. When installed the magnets must oppose each other. Do not fit the magnet into a magnetic material. Use aluminum brass or stainless steel. Use a stainless steel grub screw on the bush unit. The sensor is then mounted as close to the rotating magnet as possible. Be careful that it will not touch under vibration. A 1mm gap is suitable. The sensor has two sides. Only one side will be active. Mount it with the flat part towards the magnet. You can do an quick test with your dremmel tool by putting the bush on the dremmel and holding the sensor near it while it is turning. On version ‘c’ the unit has a special mode to test the sensor. Mode 10 allows you to test the sensor. The unit should display ‘01’ and if the sensor comes near the magnet it must display ‘00’. Should you need another magnet it is a ‘Rear Earth’ magnet 5mm diam. and 1,5mm thick. Spare magnets and sensors can be purchased from your supplier.

Your Rev counter display has a thin plastic protective cover on the display it can be removed for clearer view.

On/Off switch.

This switch will switch the unit on only. It will switch off automatically if no input is received for 4 minutes or if you select 40 on the mode switch.

Note that when you select 10 on the mode switch the unit will not switch off.

Mode switch.

The mode switch on this model has four functions. During switch on the unit will display the gear ratio. It then reverts to max RPM after 3 seconds.

If you now press the mode switch and keep it pressed it will display 30 this is the Max display mode.

If you keep the button pressed it will display 20 this is the average mode.

If you keep the button pressed it will display 10 this is the test mode.

If you keep the button pressed it will display 40 this is the power down mode.


Operation is simple. The unit will indicate the gear ratio at the time of switch on. Press the mode button to alter. Gear ratios of ,5 to 2,5 can be achieved. To find the gear ratio which your model has, Count the teeth on the output shaft’s gear and divide it by the number of teeth of the gear on your engine. Eg the drive shaft has 31 teeth and the engine has 20 then the ratio will be 1.55. Now set the rev counter to 155 by pressing the button. The number will roll over at 255. For a straight drive or measuring direct on the engine set the unit to 100.

Once the gear ratio is set the unit will automatically store it after three seconds. The next time the unit is powered it will display the gear ratio which was set up. The unit will now indicate a value between 3 and 7. This is the minimum value that the unit can measure. Remember that all readings are multiplied with 100 thus 3 to 7 actually indicates 300 to 700 RPM.

The default mode of the unit is set to RPM maximum. Thus when you test it with your dremmel tool it will jump to the RPM of the Dremmel and remain there. Press the mode button now until it displays ‘20’ and release it again. The unit will then indicate the average RPM. Test it with your dremmel again. It will now indicate the RPM of the dremmel as you speed it up and down. Pressing the mode button again until ‘10’ is displayed puts it into the test mode and keeping the button pressed until ‘30’ is displayed the unit will revert to maximum and will display the maximum RPM of the previous test.

Using the RPM counter in a Boat (as an example.) .Connect the sensor. Switch the RPM meter on. Check that it indicates the correct gear ratio for 3 seconds. Remember it is set for maximum readout. Put the boat on the water and drive up and down be careful not to jump out of the water. Bring the boat in and do not rev engine again. The Rev counter will show the maximum RPM achieved on the water. You can now put the boat back on the water be sure to bring the RPM down to slow speed below 18000 RPM. The Rev counter will now automatically reset the maximum RPM and find a new maximum reading. Each time you slow down, the maximum reading from the previous acceleration is shown and each time you accelerate from below 18000 RPM to above 18000 RPM the reading is reset. Thus revving the motor will continually reset the reading.


Display:                                                LCD

Number of digits:                                  3

Reading in RPM:                                  x 100

Resolution:                                            + - 100 RPM.

Maximum RPM reading:                       750 x 100 = 75 000 RPM

Compensation for gear ratio:                 .5 to 2.55

Accuracy:                                             0.1%

Average:                                               20 samples

Voltage supply:                                     3.6 to 6 Volts.

Charging supply & max Voltage:               7-8Volt @ 20mA

Current consumption:                            4 mA average. @ 5.5V.


For any orders you can contact Myself directly.

PS: This is the new version release. (2001 version)

Compiled on 29 December 1998 by G Baldwin

Last Updated on 30 October, 2001

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