DIGITAL RPM METER K2625

Product Overview

This digital RPM meter is designed for two and four-stroke petrol engines.

Key Features:

• Range: 100 to 9900 rpm
• Displays hundreds and thousands of rpm (x100 rpm).
• PVC frontplate included.
• Contact debouncing circuit.
• Easy calibration.
• Power supply: 10 to 15VDC/200mA.

Advantages and Description

This tachometer can be adapted to any car or motorcycle equipped with a petrol engine, regardless of the number of cylinders. Unlike conventional tachometers, it uses two displays to show the number of revolutions per minute (r.p.m.) numerically.

Key Advantages:

• Easy reading
• Small size
• No reading error
• Robustness

The displays show the hundreds and thousands of the measured value. The unit is compact and easy to house, with a frontplate included.

Technical Data

• Power supply: 10...15VDC, not stabilized
• Supply current: maximum 200mA
• Technology: CMOS
• Resolution: 100 r.p.m.
• Scale: 100...9900 r.p.m.
• Displays: 1/2" LEDs (static)
• Voltage stabilizer on the print
• Input sensitivity: minimum 3V; maximum 20V
• Input impedance: typ 30K
• Simple calibration
• Adjustable brightness
• Contact bounce suppression

Construction

The kit consists of two separate printed circuit boards: the display print (P2625A) and the base print (P2625B). The display print should be constructed first, followed by the base print.

Displayprint Construction:

Mount 7 resistors R12 (390 Ohm) and 7 resistors R13 (390 Ohm). Mount a 16-pin IC socket for IC5 and IC6. Mount both displays, ensuring the decimal point is oriented correctly. Install IC5 and IC6 (CD4511 type or equivalent), noting the notch orientation. Connect flat cables to the base print.

Baseprint Construction:

Mount straps J1-J4. Mount resistors R1-R11 with specified values and colors (e.g., R1, R2: 15K Ohm brown-green-orange). Mount trimmers RV1 (100K) and RV2 (47K). Mount small signal diodes D1 and D2 (1N914 or 1N4148), paying attention to polarity. Mount zener diode ZD1 (20V), checking polarity. Mount capacitors C1-C12 with specified values and types (e.g., C1-C4: 100nF Sibatit, C10: 10uF electrolytic), checking polarity for electrolytic capacitors. Mount voltage regulator VR1 (UA7808 or equivalent), ensuring its metal side is flush with the board. Mount IC sockets for IC1-IC4. Mount transistor T1 (BC547 or equivalent). Mount terminals for power and input. Install IC1 (CD4518), IC2 (CD4093), IC3 (CD4098), and IC4 (NE555), noting notch orientations. Connect the wires from the display print to the base print, ensuring wire "1" connects to hole "1".

Assembly Diagram Description:

Figure 1 illustrates the mechanical connection of the display print and base print. It shows two boards being joined with screws passing through aligned holes, with nylon spacers inserted between them.

Usage

Connecting the unit to the engine is straightforward. Connect the power supply to the "+" and "-" terminals, corresponding to the battery's "+" and "-". Ensure the +V supply is taken after the ignition switch so the unit is powered off when the engine is stopped. Connect the "IN" input to the positive terminal of the breaker (the connection point between the breaker and the coil). The existing condenser at this point can remain connected.

When the engine starts, the number of revolutions will be displayed.

Note on Electronic Ignition Systems:

For some vehicles with electronic ignition systems, the meter might display zero at high speeds. In such cases, fit a 10K trimmer parallel to the zener diode (RV3 in the schematic diagram) and adjust it until the meter functions correctly at all speeds.

Testing and Setting

After assembly, recheck component placement. Connect a 10-15V power supply to the "+" and "-" points. The "IN" connection can remain open initially. The displays should show "00" regardless of the trimmer RV2 position. The display brightness can be adjusted using trimmer RV1.

Testing and setting can be performed using either a transformer (50 or 60 Hz calibration source) or an audio generator.

1. Setting with a Transformer:

Use a transformer with a 3-15V secondary voltage (current is not critical). Connect the transformer output to the "-" and "IN" points. Adjust trimmer RV2 according to Table 1 to match the displayed value with a specific engine type and cylinder count. The display shows hundreds and thousands of RPM.

2. Setting with an Audio Generator:

Use an accurate audio generator set to exactly 50Hz. The output signal can be sine or square wave. Connect the generator output to the "-" and "IN" points. The signal must be at least 3V. Adjust trimmer RV2 according to Table 1 to match the displayed value with a specific engine type and cylinder count.

Function Description

The input voltage is stabilized by voltage regulator VR1 to provide 8V for the circuit. Capacitors C1 and C2 provide high-frequency de-coupling. The input frequency is applied to "IN" and fed to the base of transistor T1 via resistors R1 and R2. Zener diode ZD protects T1 from high input voltages, and C12 acts as a low-pass filter. A square wave with the input signal's frequency is generated at T1's collector. This signal drives IC2 (a monostable multivibrator) to produce a fixed-width pulse, independent of the input frequency or width. This pulse is sent to the counting input of IC1 (a double BCD counter). The BCD outputs of IC1 are connected to IC5 and IC6 (BCD to 7-segment decoders), which control the displays via resistors R12 and R13.

An oscillator built around IC4, adjusted by RV2, generates a frequency. Its output pulses the first monostable multivibrator (MMV) in IC3, creating a small pulse at IC3's output 7. This pulse is added to the latch input (pin 5) of IC5 and IC6, transferring the BCD data to the displays. Once the pulse disappears, the displays hold the last read value. Output 7 of IC3 also triggers the second MMV in IC3. This MMV produces a pulse at output 10, which resets the BCD counter (IC1) via its reset inputs. RV2 is adjusted so that IC4 generates a pulse when the counter reaches the desired value for a known input frequency.

A separate part of IC2 is used as an oscillator with its duty cycle controlled by RV1. This frequency is applied to the blanking inputs (pin 4) of IC5 and IC6, allowing brightness control of the displays.

Table 1: Calibration Settings

Table 1 provides calibration settings for different engine types (2-stroke/4-stroke) and cylinder counts, based on either a 50Hz or 60Hz calibration signal. The asterisk (*) denotes the most current type of engine.

* = most current type of engine