A simple method would be to make a coil of wire around one of the spark plug leads which goes to a digital comparator. This would generate a nice square wave signal which is perfect for measuring RPM.
Each time the signal pulses, you have a rotation, since the shift from bottom dead center to top dead center of one piston occurs after a full revolution.
After you have a digital output, you need some device to measure the pulse and display a numerical value. The easiest way to do this is to use a binary counter circuit which counts the amount of pulses measured over a period of time and constantly refreshes this value to a numerical display.
I'm not sure where you'll find examples of this; however, I could probably come up with a good little circuit to do the job.
Each time the signal pulses, you have a rotation, since the shift from bottom dead center to top dead center of one piston occurs after a full revolution.
After you have a digital output, you need some device to measure the pulse and display a numerical value. The easiest way to do this is to use a binary counter circuit which counts the amount of pulses measured over a period of time and constantly refreshes this value to a numerical display.
I'm not sure where you'll find examples of this; however, I could probably come up with a good little circuit to do the job.
Just buy one! Seriously! I used to own a petrol powered car many years ago and bought an aftermarket RPM analog meter for it. Only 3 connections; to switched power, ground and a pickup point at the coil (this was an old engine, way before EFI). Worked for 5 years until I sold the car. RPM meter cost around USD 25.
EDIT: Meter made in Taiwan, AutoGauge brand, I think.
EDIT: Meter made in Taiwan, AutoGauge brand, I think.
txt file:
Repair Your Own Rev-counter
Personally, I like to see the old style dials in a cobra. The trouble with them is that it's such a pain getting the rev counter converted to a V8, let alone getting the Speedo sorted. OK, I know they can be sent away and for a charge of £50 or so, they can be sorted, but there's very little to sorting it out yourself for under £10. All you need are a few tools; soldering iron, solder, a pair of side cutters, mulitmeter and a few cheap components from Maplin.
Basically, most, older electronic rev-counters are 0-10 mA ammeters. This means that when a current flows through the dial's coil, the needle moves up the scale linearly with the current. So at 5mA the dial will be showing half its Full Scale Deflection (FSD). E.g. 10mA = FSD = 7000 rpm therefore 5mA = 3500 rpm.
The next part to consider is the input. In most cases, this comes from the coil's (-ve) terminal. But some come from an auxiliary output within the ignition amplifier. E.g. MSD, etc. The pulse from a coil's (-ve) terminal will be around 250V, were as the pulse from an MSD tacho output is 12V. With such a large difference, in these two voltages, it's important to know what you're going to connect it to. See Note on Circuit Diagram
Obviously, the frequency of these pulses is dependent on the speed the engine is turning at. So, bearing in mind that only half the cylinders fire for every revolution of the engine, the frequencies at 1000 rev intervals is as follows:
Revs 8 cylinder 6 Cylinder 4 Cylinder
1000 rpm 66.6 HZ 50 HZ 33.3 HZ
2000 rpm 133.3 HZ 100 HZ 66.6 HZ
3000 rpm 200 HZ 150 HZ 100 HZ
4000 rpm 266.6 HZ 200 HZ 133.3 HZ
5000 rpm 333.3 HZ 250 HZ 166.6 HZ
6000 rpm 400 HZ 300 HZ 200 HZ
7000 rpm 466.6 HZ 350 HZ 233.3 HZ
Now all we need is something to convert these pulses into a 0-10 mA current. The circuit is based around a Frequency to Voltage Converter (LM2917N).
For the more technically minded, the input (Pin 1) needs to swing around the 0 volt reference, hence the use of R4, C1 and R2. The time constant is set by RV1 and C2. This sets the output voltage, depending upon the input frequency. (Vout = Fin x Vcc x R1 x C1), where Vcc = 7.65V. RV2 is used to limit the current through the dial. Pins 6,7,13 and 14 are not used on this IC.
Before you start to build the circuit, take the dial to bits. This is done normally by removing the screws from the back and unscrewing the front facia. Inside it, you will find several wires and a P.C.B. board. Disconnect the wires to the board and remove it. (See Pic1). Now locate the wires that go to the coil of the dial. One will probably be red or orange (+), the other black (-ve).Using a 6V supply, wire the dial's coil in series with the variable resistor, RV2. You can use 4 x 1.5V batteries for the supply. Adjust the resistor so as the dial goes to its maximum reading. E.g. 7000 rpm. You have now set the current limit up for the circuit, so do not adjust the resistor after doing this!
The next resistor to set up is RV1. Use a multimeter and adjust as follows:
0-7000rpm Rev Counter
76,500ohms, (76.5K) for an 8 cylinder
101,800ohms (101.8K) for a 6 cylinder
152,800ohms (152.8K) for a 4 cylinder
0-6000rpm Rev Counter
89,100ohms (89.1K) for an 8 cylinder
118,800ohms (118.8K) for a 6 cylinder
178,200ohms (178.2K) for a 4 cylinder
The formula for this is: R1 = Vout / Vcc x Fin x C2
R1 = 6V / 7.65V x Fin x 0.000000022
Thumb nail of circuit
Thumb nail of a Rev-counter
Components
RV1 100K Variable resistor PCB Mount (200 K ohms for anything less than 8 cylinder)
RV2 1K Variable resistor PCB Mount
IC 1 LM 2917N (Maplin P/N WQ38R)
R1 10 K Ohm Resistor
* R2 20 K Ohm Resistors
R3 470 Ohm Resistors
* R4 200 K Ohm Resistor
D1 IN4001 Diode
C1 100nF 250V Capacitor
C2, C3 22nF Capacitor
C4 1uF Capacitor
* Note: If using the 12 Volt signal from an MSD ignition, link out R4 and replace R2 with a 430 K Ohm Resistor.
Once the circuit is built, solder the dial's coil wires, the input wire and the power wires onto the board. Glue the board in place; reassemble the dial and its ready to be fitted into the dash.
Repair Your Own Rev-counter
Personally, I like to see the old style dials in a cobra. The trouble with them is that it's such a pain getting the rev counter converted to a V8, let alone getting the Speedo sorted. OK, I know they can be sent away and for a charge of £50 or so, they can be sorted, but there's very little to sorting it out yourself for under £10. All you need are a few tools; soldering iron, solder, a pair of side cutters, mulitmeter and a few cheap components from Maplin.
Basically, most, older electronic rev-counters are 0-10 mA ammeters. This means that when a current flows through the dial's coil, the needle moves up the scale linearly with the current. So at 5mA the dial will be showing half its Full Scale Deflection (FSD). E.g. 10mA = FSD = 7000 rpm therefore 5mA = 3500 rpm.
The next part to consider is the input. In most cases, this comes from the coil's (-ve) terminal. But some come from an auxiliary output within the ignition amplifier. E.g. MSD, etc. The pulse from a coil's (-ve) terminal will be around 250V, were as the pulse from an MSD tacho output is 12V. With such a large difference, in these two voltages, it's important to know what you're going to connect it to. See Note on Circuit Diagram
Obviously, the frequency of these pulses is dependent on the speed the engine is turning at. So, bearing in mind that only half the cylinders fire for every revolution of the engine, the frequencies at 1000 rev intervals is as follows:
Revs 8 cylinder 6 Cylinder 4 Cylinder
1000 rpm 66.6 HZ 50 HZ 33.3 HZ
2000 rpm 133.3 HZ 100 HZ 66.6 HZ
3000 rpm 200 HZ 150 HZ 100 HZ
4000 rpm 266.6 HZ 200 HZ 133.3 HZ
5000 rpm 333.3 HZ 250 HZ 166.6 HZ
6000 rpm 400 HZ 300 HZ 200 HZ
7000 rpm 466.6 HZ 350 HZ 233.3 HZ
Now all we need is something to convert these pulses into a 0-10 mA current. The circuit is based around a Frequency to Voltage Converter (LM2917N).
For the more technically minded, the input (Pin 1) needs to swing around the 0 volt reference, hence the use of R4, C1 and R2. The time constant is set by RV1 and C2. This sets the output voltage, depending upon the input frequency. (Vout = Fin x Vcc x R1 x C1), where Vcc = 7.65V. RV2 is used to limit the current through the dial. Pins 6,7,13 and 14 are not used on this IC.
Before you start to build the circuit, take the dial to bits. This is done normally by removing the screws from the back and unscrewing the front facia. Inside it, you will find several wires and a P.C.B. board. Disconnect the wires to the board and remove it. (See Pic1). Now locate the wires that go to the coil of the dial. One will probably be red or orange (+), the other black (-ve).Using a 6V supply, wire the dial's coil in series with the variable resistor, RV2. You can use 4 x 1.5V batteries for the supply. Adjust the resistor so as the dial goes to its maximum reading. E.g. 7000 rpm. You have now set the current limit up for the circuit, so do not adjust the resistor after doing this!
The next resistor to set up is RV1. Use a multimeter and adjust as follows:
0-7000rpm Rev Counter
76,500ohms, (76.5K) for an 8 cylinder
101,800ohms (101.8K) for a 6 cylinder
152,800ohms (152.8K) for a 4 cylinder
0-6000rpm Rev Counter
89,100ohms (89.1K) for an 8 cylinder
118,800ohms (118.8K) for a 6 cylinder
178,200ohms (178.2K) for a 4 cylinder
The formula for this is: R1 = Vout / Vcc x Fin x C2
R1 = 6V / 7.65V x Fin x 0.000000022
Thumb nail of circuit
Thumb nail of a Rev-counter
Components
RV1 100K Variable resistor PCB Mount (200 K ohms for anything less than 8 cylinder)
RV2 1K Variable resistor PCB Mount
IC 1 LM 2917N (Maplin P/N WQ38R)
R1 10 K Ohm Resistor
* R2 20 K Ohm Resistors
R3 470 Ohm Resistors
* R4 200 K Ohm Resistor
D1 IN4001 Diode
C1 100nF 250V Capacitor
C2, C3 22nF Capacitor
C4 1uF Capacitor
* Note: If using the 12 Volt signal from an MSD ignition, link out R4 and replace R2 with a 430 K Ohm Resistor.
Once the circuit is built, solder the dial's coil wires, the input wire and the power wires onto the board. Glue the board in place; reassemble the dial and its ready to be fitted into the dash.
The next:
similar, but different:
Txt-file:
AUTOMOTIVE TACHOMETER (the schematic is named tach.jpg)
Circuit for an automobile tachometer using the LM2907 or LM2917 frequency to
voltage converters. The LM2917 has a regulator onboard. The circuit is the same
for 4,6 or 8 cylinder engines. However, the component values are different. On a
gasoline powered engine, the pickup (input) comes from the distributor. Most
distributor caps have a tap-off that can be used. The circuit can be adopted for use
with a diesel engine. On the diesel engine, tap into the varying A.C. voltage
coming off the alternator. The equations used with this chip are:
Vo=Vcc x Fin x R1 x C1: where Vo=output voltage, Vcc= supply voltage,
Fin=input frequency in hertz, R1=resistance of R1, C1=capacitance of C1
and RPM=(120 x freq.) / (# of cylinders): to find the input frequency depending on
the engine RPMs and the number of cylinders.
DESCRIPTION & CONSTRUCTION
Design of circuit for a four cylinder gasoline engine.
RPM Freq., Hz Io(uA)
1000 33.3 17
2000 66.6 33
3000 100 50
4000 133 67
5000 166 83
6000 200 100
Using a 100uA meter movement and the 14 pin LM2917 package, the following
component values are used. For the LM2917 Vcc=7.5V when the voltage
regulator is used.
Calculate the value of R1 x C1, and pick a common value for the capacitor. In this
case, .01uF was picked. This makes the value for R1=360K. The capacitor(.1uF) in
parallel with R1 is used to damp the meter. This may have to be changed
depending on the meter movement. The full scale meter deflection(100uA in this
case) is set using the 10K pot in series with the 39K resistor connected to
pin 5. The input frequency is divided and filtered prior to input to pin 1. These
values may have to be changed dependent on the specific automobile engine. All
resistors are half watt. Capacitors are non-polarized disc, mylar or tantalum. The
diode is 1N4001.
Construct the circuit on a vector board. Ensure the potentiometer is placed for easy
access. The entire circuit can be fitted to the input terminals of the meter. This
makes for a clean, compact tachometer that can be mounted anywhere.
CALIBRATION
Calibrate the meter before installation using a pulse generator. Set the generator to
the maximum input frequency, 1.5 mS pulse width, 10 V amplitude. Adjust the
10K pot to get 100uA meter reading. Check all frequencies for the correct output
current.
similar, but different:
Txt-file:
AUTOMOTIVE TACHOMETER (the schematic is named tach.jpg)
Circuit for an automobile tachometer using the LM2907 or LM2917 frequency to
voltage converters. The LM2917 has a regulator onboard. The circuit is the same
for 4,6 or 8 cylinder engines. However, the component values are different. On a
gasoline powered engine, the pickup (input) comes from the distributor. Most
distributor caps have a tap-off that can be used. The circuit can be adopted for use
with a diesel engine. On the diesel engine, tap into the varying A.C. voltage
coming off the alternator. The equations used with this chip are:
Vo=Vcc x Fin x R1 x C1: where Vo=output voltage, Vcc= supply voltage,
Fin=input frequency in hertz, R1=resistance of R1, C1=capacitance of C1
and RPM=(120 x freq.) / (# of cylinders): to find the input frequency depending on
the engine RPMs and the number of cylinders.
DESCRIPTION & CONSTRUCTION
Design of circuit for a four cylinder gasoline engine.
RPM Freq., Hz Io(uA)
1000 33.3 17
2000 66.6 33
3000 100 50
4000 133 67
5000 166 83
6000 200 100
Using a 100uA meter movement and the 14 pin LM2917 package, the following
component values are used. For the LM2917 Vcc=7.5V when the voltage
regulator is used.
Calculate the value of R1 x C1, and pick a common value for the capacitor. In this
case, .01uF was picked. This makes the value for R1=360K. The capacitor(.1uF) in
parallel with R1 is used to damp the meter. This may have to be changed
depending on the meter movement. The full scale meter deflection(100uA in this
case) is set using the 10K pot in series with the 39K resistor connected to
pin 5. The input frequency is divided and filtered prior to input to pin 1. These
values may have to be changed dependent on the specific automobile engine. All
resistors are half watt. Capacitors are non-polarized disc, mylar or tantalum. The
diode is 1N4001.
Construct the circuit on a vector board. Ensure the potentiometer is placed for easy
access. The entire circuit can be fitted to the input terminals of the meter. This
makes for a clean, compact tachometer that can be mounted anywhere.
CALIBRATION
Calibrate the meter before installation using a pulse generator. Set the generator to
the maximum input frequency, 1.5 mS pulse width, 10 V amplitude. Adjust the
10K pot to get 100uA meter reading. Check all frequencies for the correct output
current.
Attachments
I have an old rpm meter from my Mercedes W123. It uses one 8-pin i.c. SAK 215. It is made in Germany by VDO. I suspect the i.c.is faulty. I could not locate any electronic parts vendors selling this particular i.c. in Malaysia, Singapore, India and the US (New York).
Anyone has any idea as to where I could get hold of one of this i.c.? One vendor in US offered to order provided I am buying 1000
pieces!
Shan.
Anyone has any idea as to where I could get hold of one of this i.c.? One vendor in US offered to order provided I am buying 1000
pieces!
Shan.
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