Traffic indicator works from 12V or 24V

This circuit is a lot more complicated than the conventional thermo-mechanical switch unit used in cars but has been included to show the amount of logic required to provide the same function.

Three optical interrupters are operated by a slotted disc attached to the steering wheel. When blocked, their collectors are held high by the 10kW resistors and because the outputs of IC5a & IC5d are normally high, the inputs to the gates of
IC6a, IC6b, IC6c & IC6d are high, so that their outputs are low. The outputs of IC4b & IC4c are also low; hence the output of IC3c is low.

This makes all the inputs to IC1d & IC1b low so that their outputs are high. Because one input of IC1a & IC1c is now high, then their outputs are low. With both inputs of IC3c low then its output is low. The same situation applies to IC3a and the indicators (relays) are off.

When the indicator switch is operated, one input to IC5d (or IC5a) goes high so that its output goes low. This means that one input to both IC6c & IC6d (or IC6a & IC6b) goes low, sending their outputs high and therefore one input of both IC2a & IC2c high, making their outputs low. With both inputs of IC3a now low, its output is also low, effectively turning off the indicator lights for the other direction if they happen to be on.

Cancel pushbutton

The cancel pushbutton (switch S1) works the same way, in that it pulls the other input of IC5d (IC5a) high. The operation of the indicator switch also sends one input of IC3c (or IC3a) high so that its output goes high. In turn, one input of both IC1b & IC1d goes high, so that both their outputs go low.

This puts all the inputs of both IC1a & IC1c low, making their outputs high. This makes the inputs of IC3c high and so its output goes high, turning on the indicator lights via transistor Q2 and RLY2. Pin 5 of IC1b also goes high, keeping its output low. Also the input to IC4c goes high, making its output high to keep one input of IC1d high and its output low. So both IC1b & IC1d stay in this state when the indicator switch is released (open) so that the indicator lights stay on.

There are latches to make the optical interrupters directional. If the steering wheel operates Opto1 first, then IC6a’s output goes high (one input low) making one input of IC1c high and it’s output low but the lights stay on via IC1a, IC3d & IC1b.

When the steering wheel operates Opto2, the output of IC6b goes high, making the input of IC1a high and its output low. Now with both inputs of IC3c low then its output goes low and turns the lights off. If Opto2 is operated first, as when the wheel is turned in the direction you are indicating to turn, then IC6b’s output goes high, sending one input of IC1a high so that its output goes low, but this does not affect IC3c. As its other input is high, then its output stays on.

By the time the wheel operates Opto1, Opto2 will have been block-ed again so that IC1a returns to the same state as before, keeping the indicators going. However the output of IC1c will be made low by the output of IC6a going high. In turn, the output of IC4c goes low, in turn making one input of IC1d turn low, sending its output high so that the output of IC1c stays low even after Opto1 is blocked again.

IC4d allows for any number of steering wheel turns. In this case, when the wheel goes all the way around, then Opto3 is operated, to make one input of IC4b high. This together with the other input being made high by IC3c means that its output is made high. This makes one input of IC3d high, doing the same job as the indicator switch and so resetting the latch that uses IC1c. The other direction works in exactly the same way.

Only one optical interrupter can be unblocked at a time. The centre interruptor (Opto2) must be unblocked when the steering wheel is in the straight ahead, while the other optos are placed equally on either side. They need to be placed so that they provide cancellation when the steering wheel is turned back by a reasonable amount; eg, after exiting a corner.

Indicator switch

The indicator switch (S2) consists of a piece of thin tube pivoted at one end with a couple of compression springs to keep it central and with a microswitch placed on either side so that one is closed when the lever is pushed up and the other when the lever is pushed down. A rod inserted into the tube, with a knob on it, operates a third microswitch when it is pushed in, to act as the cancel button (S1).

While the relays select the left or right-hand indicator lights, the flashing function is performed a multivibrator based on transistors Q3 & Q4. Q4 is connected in Darlington fashion to Q5 and power transistor Q6 which then drives the indicator lamps. Diodes D3 and D4 feed the same 24V through to the piezo buzzer (which has an inbuilt oscillator) via the 680W current limiting resistor.

The circuit may be run from 12V by omitting the 7812 regulator and changing the relays to 12V types.

Ron Groves,

Fig.1: quite a lot of logic circuitry is required to duplicate the functions of a conventional thermo-mechanical traffic indicator.