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G8MNY > TECH 09.11.26 16:52l 114 Lines 6020 Bytes #5 (0) @ WW
BID : 10640_GB7CIP
Read: GUEST
Subj: Car Dwell Angle & RPM Meter
Path: JH4XSY<N3HYM<K5DAT<VE2PKT<PY2BIL<GB7CIP
Sent: 261109/0748Z @:GB7CIP.#32.GBR.EURO #:10640 [Caterham Surrey GBR]
From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
By G8MNY (Updated Jan 09)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
This old device (from an old mag article) is only suitable for petrol cars with
standard distributor contact breakers. Just 3 leads are needed 12V, 0V, and the
Contact Breaker. It give 4 ranges 0-20 DC, 0-2k RPM, 2-6k RPM, & Dwell time
(90-0 4cyl & 60-0 6cyl), although it could easily be calibrated for any range.
It uses a reg & 2 74 ICs, a Hex inverter & a monostable timer.
+12V 1N4001 ____
>ﾄﾂﾄ33Rﾄｴ>ﾃﾄﾄﾂﾄｴ7805ﾃﾄﾂﾄﾄﾄﾂﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾂﾄﾄﾄﾄﾄﾄﾂﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾂﾄ>+5V
ｳ +ｳ ﾀﾄﾂﾄﾄﾙ ｳ 560R R 27K ﾚﾄﾄﾄﾁﾄﾄﾄｿ 470
33K === ｳ === _ｳ_ Red ﾃﾄﾄｴ11 14 ｳ ｳ
ｳCal 100uｳ ｳ u1ｳ \_/=LED C === ｳ ｳ 1N4148 ｳ
3K3 ﾁﾄﾄﾄﾁﾄﾄﾄﾄﾁﾄﾄﾄﾁﾄﾄ0V u22ﾀﾄﾄｴ10 6ﾃﾄｴ>ﾃﾄﾄﾂﾄﾄﾄﾄﾄﾄｿ ｳ
ｳ Volts to ｳ\ ｳ 74121 ｳ ｳ ｳ ｳ
ﾀ>Meter SW+ +5V 3ｳ \ 4 ｳ MONO ｳ 5K 5K 200R
Diodesﾚｴ>ﾃﾄﾄｴ ﾚﾄｴ7405>oﾄﾄｴ5 ｳ ﾚ>2000 ﾚ>6000 ﾚ>6000
1N4148ｳ ｳ\ｳ14 ｳ ｳ / ﾚﾄｴ4 ｳ ｳ RPM ｳ RPM ｳ Zero
CB ﾚ ｳ 1ｳ \ ｳ ｳ/ ﾃﾄｴ3 7 ｳ Meterｳ Meterｳ Meterｳ
>ﾄ3K3ﾄﾂﾄｴ<ﾃﾄ3K3ﾄﾂﾄﾄﾂﾁﾄﾂﾄｴ7405>oﾄｴ ｳ ﾀﾄﾄﾄﾂﾄﾄﾄﾙ SW+ ｳ SW+ｳ SW- ｳ
0.5W ｳ 5V ｳ 3K3_ｳ_ｳ / 2ｳ 0Vﾄﾁﾄﾄﾄﾄﾄﾁﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾁﾄﾄﾄﾄﾄﾄﾁﾄﾄﾄﾄﾄﾄﾙ
=== === ｳ /_\ｳ/ｳ7 ｳ ｳ\ ｳ\ ｳ\
47nｳ 68nｳ ｳ ｳ ｳ ｳ ｳ \ 11ｳ \ 10 5ｳ \ 6
>ﾄﾄﾂﾄﾄﾁﾄﾄﾄﾄﾄﾄﾄﾄﾄﾁﾄﾄﾁﾄﾄﾁﾄﾄﾄｴ ﾀﾄﾄｴ7405>oﾄﾄﾄｴ7405>oﾄﾄﾄﾂﾄﾄｴ7405>oﾄｿ
-veｳ INPUT AMP 0V 13ｳ / 12 ｳ / ｳ ｳ / ｳ
ｳ ____ ｳ/ ｳ/ /ｳ ｳ ｳ/ 2K7
ｳ -/.6mA\+ / ｳ9 ｳ ｳ
ﾃﾄo<ﾂｴMeter ﾃﾂﾄ>o< 0-20V Volts ﾚﾄﾄo<7405ﾃﾄﾄﾄｴ 5K<ﾄﾄMeter
6k-)ﾄo ｳ \____/ ｳ o< 2000-6000 RPM 68R 8 \ ｳ 27R DWELL SW+
ﾃﾄo ｳ + ｳ o< 0-2000 RPM _ｳ_ \ｳ _ｳ_ ｳ
ﾀﾄo ﾀﾄﾄﾄｴﾃﾄﾄﾄﾙ o< Dwell Open\_/=yel Close\_/=gn ｳ
220u 0Vﾄﾁﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾁﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾙ
HOW IT WORKS
The 12V is regulated to 5V for the logic to maintain meter calibration (with a
different regulator a 6V version could made.) A calibrated meter supply
resistor (33K + 3K3 for my meter!) gives the voltage feed to the meter switch.
ｳ Coil
Coil 300Vﾞ Resonance The high voltage spiky contact
Voltage Primaryﾞ breaker input is filtered,
12V___ back EMFｳﾝ_____ offset voltage applied
ｳ pulse ｳｲ (for 12V) with the series 5V
Open ｳ__________________ｳｱ Open zener the 3 Rs, refiltered,
OV Closed ﾝ then clipped before being
DWELL (ON) TIME ｳ buffered by the 1st hex amp.
Spark
Buffer This gives a faithful
Output __________________+5V inverted square wave output
______ｳ ｳ______ that represents the actual
contact state.
This is re-inverted and fed to trigger the monostable IC to remove the on - to
off ratio. The monostable's CR time is set to be a bit faster than the fastest
6000 RPM (10mS), so the resultant square wave output's on to off time varies
with the engine revs.
ﾚﾄｿ ﾚﾄｿ ﾚﾄｿ ﾚﾄｿ ﾚﾄｿ ﾚﾄｿ ﾚﾄｿ ﾚﾄｿ ﾚﾄｿﾚﾄｿ
Mono ｳ ｳ ｳ ｳ ｳ ｳ ｳ ｳ ｳ ｳ ｳ ｳ ｳ ｳ ｳ ｳ ｳ ｳｳ ｳ
Outputﾙ ﾀﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾙ ﾀﾄﾄﾄﾄﾄﾄﾄﾄﾄﾙ ﾀﾄﾄﾄﾄﾄﾄﾙ ﾀﾄﾄﾄﾄﾄﾙ ﾀﾄﾄﾄﾄﾙ ﾀﾄﾄﾄﾙ ﾀﾄﾄﾙ ﾀﾄﾙ ﾀﾙ ﾀ
Low Revs High Revs
Mean _____.......----~~~
DC _________________............--------ﾄﾄﾄﾄﾄﾄﾄﾄﾄ~~~~~~~
This is the fed via a diode to lose any IC "off" DC, to the meter via 2 presets
one with a -ve meter offset to give the 2 linear RPM scales. The meter is
heavily damped with a 220uF, to give a steady reading at low Revs.
The dwell angle feed, uses up the rest of the Hex amps to further buffer the
signal and drives 2 LEDs to show the contact status, and also the dwell meter
feed via a calibration pot.
Coil With contact breaker ignition it
Current _..--ﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄｿ is important not to have to short
0A___.ﾄ~ ^ 5Amp ｳ____ a Dwell time as the coil takes time
|Charge| to build up to full current and
OFF | DWELL ON | OFF "charge up" its magnetic circuit.
Typical recommended 4 cylinder values are 33-45% ON times depending on the
maximum engine RPM and ignition coil type. So called "Sports" types are really
6V coils and an external 100W resistor to safely limit the stationary current
(10A!) to give faster charge times for higher revs, then came along electronic
ignition with its 300V via SCR to coil DC pulses. :-)
METER SCALES
The meter scales are quite linear, the Dwell scales are reversed and do not
start at 100% !
Deflection Volts 2K RPM 2-6K RPM DWELL 4Cy DWELL 6Cy
0% 0 0 2000 90% 60%
25% 5 500 3000 67ｫ% 45%
50% 10 1000 4000 45% 30%
75% 15 1500 5000 22ｫ% 15%
100% 20 2000 6000 0% 0%
Why there are different Dwell scales for 4 & 6 cylinders I do not know!
CALIBRATION
Using a square wave astable source like 555 with a frequency counter and
ordinary analogue meter you can mimic the engine pulse on the workbench while
varying the frequency and on to off ratio. Using just a low voltage 50Hz as the
input you can only calibrate 3000 RPM it is a good enough if it is all working.
Why don't U send an interesting bul?
73 De John, G8MNY @ GB7CIP
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