SMALL CIRCUITSCOLLECTION

017 Universal Rev Counter

5V 1: 50Hz n = 3000
S3
2: 100Hz n = 5000 R14 P4
C13 14
3: 200Hz n = 12000 3k9
C10 IC3 1k 1
D S2b
100n 7
7 R13 P3
3 2
100n CTR14 8k6
5
4
IC2 4 2k2
5 S2a R12 P2
11 !G 6 3 50Hz 3
RCX 6 18k
9 14 1
R16 CX + 7 2
4k7
10 CT 13
4M7 RX 8
R15 15 B
9
1: n = 3000
1 9 13 5 1 M2
11 D D C9
1k

2: n = 10000
X1 4060 2
12 12 IC3b IC3a 3: n = 20000
CT=0 3 11 12 3 2
13 C C 1000µ PM-4
C12 C11 R S R S C 100µA
200Hz 100Hz
10 8 4 6
68p 56p IC3 = 4013

X1 = 3.276800MHz

IC1
0V23 5V
78L05 S1b
B n
D1 x 10 BT2
BAT49
C7 C8
R3 R4 R6 R9 R9
C6
10k

12k

1k5

1k

1k
S1a 1µ 1µ 1V4 9V
f in = 200Hz 0mA8
470µ

DVM-210
C3 C5

BT1
D
470n 470n
R11
* zie tekst
15k

C1
R1
C4 * see text
22k

P1 * siehe Text

9V
15p
T1 T2
470n
T3
C * voir texte
8mA 2k2
C2
A
470n R2 R5 R7 R8
D2
4k7

4k7

1k

-Θ
10k

1N4148 BC547B BC547B BC547B 04004 - 11

K.-H. Lorenz der engine, so adjust preset P1 until the display corresponds to
This rev counter is unusual in that it has no direct electrical this.
connection to the ignition system of the engine. It is also If a moving coil meter is used for the display, the speed
unnecessary to add a winding to an ignition lead. If the ignition measurement is divided into three ranges selected by S2. The
system on the engine is electronic then all that is necessary second set of contacts of S2 (S2a), are used to switch an appro-
for the spark impulse signal pickup is a length of wire posi- priate calibration frequency from the generator. So, with S2 in
tioned 10-30 cm from the ignition lead. For an engine with position 1 preset P4 is adjusted until 3000 r.p.m. is shown on the
magneto ignition this distance will be less. meter (i.e. full scale deflection). The same process is repeated
The original idea for this design was to make a hand held with S2 in position 2 and 3 this time adjusting P3 and P2 for
rev counter or tachometer with its own battery supply that 6000 and 12000 r.p.m. respectively.
could be used to check engine speed or to calibrate another The supply to the rev counter is provided by a 9 V battery
built-in rev counter. The good resolution of this design and its to a low-drop 5 V voltage regulator. Schottky diode D1 gives
convenience in operation mean that it is also ideal for use protection against accidental battery reversal.
when adjusting the engine idle speed. The DVM will also require its own supply (not necessary if
The display for this circuit is a commercially available DVM you are using a moving coil meter). The current consumption of
panel meter and will show engine speeds up to 19,999 r.p.m. the rev counter is approximately 8 mA, while the calibration
Alternatively a 100 µA-moving coil meter can also be used to circuit consumes substantially less and is only active during
give an analogue display. calibration when S3 is pressed.
The circuit consists of three transistors. T1 amplifies the Note that the r.p.m. referred to above is valid for a single
incoming signal while T2 and T3 perform pulse shaping. The cylinder 2 stroke engine and will need to be converted for the
resulting pulses are fed to an integrating network formed by type of engine configuration which is being measured. The
preset P1, R11 and C6; the DVM measures the DC voltage relationship between the impulse frequency and the r.p.m. of
across C6. an engine is given by the formula:
An optional built-in generator is also shown. This is used
to calibrate the rev counter and also test the display. If a DVM f = 2nc/60t
panel meter is used for the display, then the 200 Hz output of
the 4060 (IC2) at pin 3 can be connected directly to the input of Where f = Impulse frequency, n = r.p.m. c = No of cylinders,
the rev counter (point D in the diagram). 200 Hz corresponds t = type of engine (2 = 2-stroke, 4 = 4-stroke).
to an engine speed of 12,000 r.p.m. for a 2-stroke, single cylin- 004004

38 Elektor Electronics 12/2000