While it is not compulsory to have indicators
and brake lights fitted to vintage cars in Australia, I thought it wise
that my Model T should have them. Given road rage with Sydney drivers and
the fact few understand hand signals, I decided to fit signal lights others
would recognise.
I decided I'd take a different path to
everyone else and design my own flasher unit. 6V flashers are still available
from all the vintage parts suppliers, so it wasn't lack of availability
that caused me to do this.
The flasher circuit is built in a diecast box bolted onto the chassis
cross member. The terminal strips on the sides go to the lights and power,
while the DIN plug connects the indicator switch.
Advantages
An electronic version would eliminate
the heavy wiring to the indicator switch to start with and also provide
a user adjustable flash rate. Some examples of 6v indicators I've seen
seemed to suffer from an excessively long "off" period and dim lights.
As the voltage supply for the oscillator
is regulated, flash rate is constant over the normal voltage range of the
battery
Unlike thermal flashers, this design does
not depend on correct lamp wattage to function properly. It also means
it's perfect for LED lights.
LED indicator lights
The problem with trying to use LED lights
with a conventional thermal flasher is that the low resistance of the flasher's
heating element keeps the LED's lit all the time as the current drawn by
the LED's is much less than incandescent lamps. My circuit completely breaks
the supply to the lights in between flashes.
I'm not promoting LED's as I prefer to
support the dying industry of incandescent lamp manufacturing, but I know
there is a lot of interest with LED lighting on cars. It is possible to
modify commercially made 12V or 24V LED signal lights to run on 6V as the
individual LED's run on between 2 and 4V. To do the modification, you'll
first need to power up the light on it's intended voltage and measure the
current through each chain of LED's, (usually around 20-40mA) and what
the LED voltage is. The 12 and 24V lights have series/parallel wiring where
groups of several LED's are in series with one current limiting resistor.
The LED's need to be rewired so each has its own resistor. This will allow
6V operation. To calculate the resistor required, use the formula R(in
ohms)=(6-Vf)/If, where Vf is the LED forward voltage, and If is the LED
forward current. Quarter watt resistors will be adequate.
Unfortunately, there are some who refuse to believe that a 6V car electrical system works as well as a 12V one and I didn't want to provide further ammunition to their argument. Think of all the Fords, and other cars built from 1919 to 1955 all running on 6V...you'd think that 36 years would be long enough to prove there is nothing wrong with 6V. The change to 12V occurred because of higher current consumption in later cars and the current limitation of generator brushes. Had alternators and solid state rectifiers appeared on the scene earlier, or cars not being equipped with unnecessary luxuries, then 6V systems may have lasted longer.
The Circuit
It's basically a schmitt trigger oscillator
built around a 4093 CMOS quad NAND gate, which then drives one of two relays.
The contacts of these relays switch the left and right side indicator lights.
The supply for the 4093 is stabilised
at around 3.9 volts with the zener diode and 220 ohm resistor. While CMOS
logic gates are happy with supply voltages from 3 to 15, the supply does
need to be regulated so that the voltage of the car electrical system will
not affect flash rate. I like to design for a 5 to 7 volt operating range.
This allows for when the generator is charging and also if the car is stopped
and other loads are on.
Oscillator details
The way the oscillator works is very simple.
The gates are simply wired as inverters with the output fed to the input.
There is also a capacitive time constant at the input. Assume the circuit
is first powered up. The 2.2uF capacitor will be discharged, meaning 0
volts at the input. Because the gate is an inverter, the output will be
high at around 3.9V. Current flows via the 180K and 100K pot to the capacitor
which starts charging. At a certain point, the input will see a high thus
switching the output low. The 2.2uF now starts discharging through the
resistors, and so it keeps oscillating.
Unlike commercially made flasher units,
this one is adjustable. So, you can select your preferred flash rate simply
by changing the RC time constant, hence the 100K trimpot.
As the CMOS gates can only supply about
20mA, a BD140 power transistor is used to drive the relays. The diode between
emitter and collector bypasses back EMF when the relay coils turn off.
I prefer this method to the usual one of wiring the diode across the coil
as it means the transistor won't be damaged if the diode shorts, and the
transistor is also protected against reverse polarity. The back EMF is
absorbed by the power supply.
I did discover during the design that
different types of 4093 will result in different flash rates. I'm using
a CD4093BCN. If you use an MC14093BCP you'll need to increase the resistance...start
with about 1.5M, or increase the capacitor value.
Note that the 220 ohm current limiting resistor is in the negative supply to the zener diode and 4093. This is done because a PNP transistor is being used to drive the relays, and therefore the 4093 positive rail must be at the same voltage as the transistor's emitter.
Relays
These are double pole units with 6v coils. The ones I used were actually 4PDT units with a coil current of about 200mA. The indicator switch determines which coil is activated simply by completing the earth return. In the centre position neither coil is selected and the indicators are off. The second set of contacts simply switch the signal light on the indicator switch, and buzzer if used. Single pole relays could used if the signal light was fed by a two diode OR gate fed from the main switching contacts.
So that the oscillator isn't running all the time when the indicators aren't being used, there's a two diode OR gate connected to the indicator switch which completes the earth return for the 4093's supply. I used 1N914's in view of the low current but just about anything can be used.
Hazard Lights
I haven't shown it in the circuit diagram, but this design makes it convenient to add a hazard light switch. Simply add a small DPST switch in parallel with the indicator switch with the common contacts connected together and earthed. Of course a SPST switch could be used with the addition of two diodes instead.
Construction
The circuit was constructed on a small
piece of veroboard and placed along with the two relays in a zinc diecast
box. This was then mounted on the front chassis cross member, a convenient
hole already existing to mount the box. A bakelite screw terminal block
connects to the indicator lights and 6v supply, and a 5 pin DIN socket
used to connect to the indicator switch. Light stereo figure eight cable
was run to the switch. In the Canadian version of 1926 Model T exists a
tube running up the steering column for the horn switch wires. I used this
same tube to run the indicator switch wires...something you couldn't do
had the indicator switch been wired the conventional way carrying the full
lamp current.
The indicator switch used is a Hella 4208.
This can be ordered in from your usual parts supplier. I paid about $74
from Scott's Auto 1. Repco wanted about $128! It is a far superior product
to the metal chrome plated switch that all the repro parts suppliers sell
which is of very flimsy construction.
Have a look at the Hella 4208 switch here.
For your interest, the bulbs I use are
6V 10W as shown here.
Of course 6v bulbs are available from other suppliers as well. I use 1929-31
Model A rear lights with the dual coloured lens; red for brake and parking
and amber for indicator. The front lights are small motorcycle lamps I
got at Bendigo swap meet.
The golden rule with 6 volt electrical
systems is to run a separate earth wire from each light directly to the
negative battery terminal. Don't rely on rusty chassis connections. If
you have good connections and use wire with an appropriate current rating,
your lights will work as well as those on a 12v car.
Other voltages
The circuit is adaptable quite easily to
run on something other than 6V. It's simply a matter of selecting appropriate
relay coils and altering the 220R resistor to suit. As there is about 13mA
flowing through this resistor, the new value can be worked out thus: (supply
volts-4)/.013
Power rating of the resistor will be (.013^2)
x R in ohms.
The BD140 is happy up to about 60V so
will easily cater for 12 and 24v operation, but keep in mind the current
draw of the relay coils.
cablehack at yahoo dot com