Adaptable with most internal combustion engine vehicles
R1,R2,R19_______1K 1/4W Resistors
R3-R6,R13,R17_100K 1/4W Resistors
R7,R15__________1M 1/4W Resistors
R8_____________50K 1/2W Trimmer Cermet
R9____________470R 1/4W Resistor
R10___________470K 1/4W Resistor
R11___________100K 1/2W Trimmer Cermet (see notes)
R12___________220K 1/4W Resistor (see notes)
R14,R16________68K 1/4W Resistors
R18____________22K 1/4W Resistor
R20___________150R 1/4W Resistor (see notes)
C1,C7_________100΅F 25V Electrolytic Capacitors
C2,C3_________330nF 63V Polyester Capacitors
C4-C6___________4΅7 25V Electrolytic Capacitors
D1,D5______Red LEDs 3 or 5mm.
D2,D3________1N4148 75V 150mA Diodes
D4________BZX79C7V5 7.5V 500mW Zener Diode
IC1__________CA3140 or TL061 Op-amp IC
IC2____________4069 Hex Inverter IC
IC3____________4098 or 4528 Dual Monostable Multivibrator IC
Q1,Q2_________BC238 25V 100mA NPN Transistors
L1_____________10mH miniature Inductor (see notes)
BZ1___________Piezo sounder (incorporating 3KHz oscillator)
SW1____________SPST Slider Switch
B1_______________9V PP3 Battery (see notes)
This circuit has been designed to alert the vehicle driver
that he has reached the maximum fixed speed limit (i.e. in a
motorway). It eliminates the necessity of looking at the
tachometer and to be distracted from driving.
There is a strict relation betw1een engine`s RPM and vehicle
speed, so this device controls RPM, starting to beep and flashing
a LED once per second, when maximum fixed speed is reached.
Its outstanding feature lies in the fact that no connection is required from circuit to engine.
IC1 forms a differential amplifier for the electromagnetic
pulses generated by the engine sparking-plugs, picked-up by
sensor coil L1. IC2A further amplifies the pulses and IC2B to
IC2F inverters provide clean pulse squaring. The monostable
multivibrator IC3A is used as a frequency discriminator, its
pin 6 going firmly high when speed limit (settled by R11) is
reached. IC3B, the transistors and associate components provide
timings for the signaling part, formed by LED D5 and piezo
sounder BZ1. D3 introduces a small amount of hysteresis.
D1 is necessary at set-up to monitor the sparking-plugs
emission, thus permitting to find easily the best placement for
the device on the dashboard or close to it. After the setting is
done, D1 & R9 can be omitted or switched-off, with battery
During the preceding operation R8 must be adjusted for better
results. The best setting of this trimmer is usually obtained
when its value lies betw1een 10 and 20K.
You must do this first setting when the engine is on but the vehicle is stationary.
The final simplest setting can be made with the help of a
second person. Drive the vehicle and reach the speed needed. The
helper must adjust the trimmer R11 until the device operates the
beeper and D5. Reducing car`s speed the beep must stop.
L1 can be a 10mH small inductor usually sold in the form of a
tiny rectangular plastic box. If you need an higher sensitivity
you can build a special coil, winding 130 to 150 turns of 0.2 mm.
enameled wire on a 5 cm. diameter former (e.g. a can). Extract
the coil from the former and tape it with insulating tape making
thus a stand-alone coil.
Circuit`s current drawing is approx. 10mA. If you intend to
use the car`s 12V battery, you can connect the device to the
lighter socket. In this case R20 must be 330R.
Depending on the engine`s cylinders number, R11 can be unable
to set the device properly. In some cases you must use R11=200K
and R12=100K or less.
If you need to set-up the device on the bench, a sine or square wave variable generator is required.
To calculate the frequency relation to RPM in a four strokes engine you can use the following formula:
Hz= (Number of cylinders * RPM) / 120.
For a tw1o strokes engine the formula is: Hz= (Number of cylinders * RPM) / 60.
Thus, for a car with a four strokes engine and four cylinders the resulting frequency @ 3000 RPM is 100Hz.
Temporarily disconnect C2 from IC1`s pin 6. Connect the
generator`s output to C2 and Ground. Set the generator`s
frequency to i.e. 100Hz and regulate R11 until you hear the beeps
and LED D5 flashes. Reducing the frequency to 99 or 98 Hz,
beeping and flashing must stop.
This circuit is not suited to Diesel engines.