Lamp Good Indicator
It is most irritating when as a pedestrian people driving cars fail to indicate. Drivers expect pedestrians to guess their intended direction without indicating, and in any case it is the pedestrian's right of way across the end of a junction.
Some drivers may not realise that their indicators have failed, and although their earlier checks indicated that all their lights were in good order, they have failed during the journey. There could also be an intemittant fault, causing the lights to fail on the road, although regular testing shews they are working correctly.
Inside most vehicles, there are secondary indicators that inform the driver when the circuits are operating the indicators, but the driver is still oblivious as to whether the actual indicators are working. This circuit replaces the conventional wiring by adding the function that the repeater indicator (on the dash board) lights only when the indicators themselves are working.
Positive and Negative Earth
The shell of motor vehicles is normally connected to one of the terminals of the battery (except in the case of "plastic" vehicles) which is referred to as the "Earth" connection. In most cases, the convention of "Negative Earth" used, which is where the Earth is connected to the Negative electrode of the battery. Conversely, vehicles using "Positive Earth" have the Earth connected to the Positive electrode of the battery.
When fitting electrical equipment such as lights, a common connection is made to all the lights in the housing. Interrupting this connection in order to fit an indicator will mean that the dash-level light will show when any light is illuminated. Hence there are two possible circuits (above) for either system: the first uses a common-negative (or negative Earith) principle whereas the second uses a common-positive (or positive Earth) principle.
Circuit Theory
Both circuits follow the same principles, however. At the core of the circuit is the Lamp, which has to be re-connected in series with the diode, D1. In this configuration, the diode is used as a basic device for detecting a current. It is taken that when the lamp lights, a current passes through the filament and hence the diode; when the lamp fails, no current passes through the diode. When a current flows through the lamp/diode network (i.e. when the lamp lights), a voltage drop of 0.7V develops across the diode (due to its P-N junction, and only if it is a Silicon-Boron/Phosphorus diode), which is the exact voltage to trigger another Silicon device - in this case, a transistor.
It is here that the circuits differ. In the negative-Earth case, the Lamp-Diode junction is 0.7V lower than the supply, so a PNP type transistor is used. Conversely, in the positive-Earth case, the Lamp-Diode junction is 0.7V higher that the supply rail so an NPN type transistor is used. Note that if you want to invert your output so that an indicator lights in error, it will not suffice to exchange transistor types, you must connect the output via a second transistor in order to perform the inversion function.
It is then a simple case of inserting an LED and series resistor in the circuit in order to acquire the desired functionality. Of course, if you are using an existing lamp, then it will suffice to insert it here (replace LD1 and R1 with the second lamp).
Component Values
The values of the components used is dependent on the surrounding circuitry. The following table summarises some suggested values for components inserted into the circuits.
Values of D1 depend on the current passing through it. For a lamp, this is proportional the the wattage of the lamp. In the table that follows, the wattage is the maximum value, and should not be exceeded. If you have two or more lamps in parallel, the sum of the wattage should be used for calculation. For larger power ratings, it is better to use larger diodes than several in parallel because different diodes will have slightly different junction potentials. This could result in more current passing through one diode than the other, which could result in one diode failing, and as a result the next will fail and so on.
| D1 Type | Maximum Power of Lamp(s) |
|---|---|
| 1N4002 | < 12W |
| 1N5401 | < 36W |
| MR751 | < 72W |
The type of transistor to be used depends on the load to be used. The load in the diagram represents an LED, but a lamp indicator could also be used. In the latter case, a different transistor that can carry a larger current needs to be used. This table summarises transistor choice:
| Q1 Type (neg-Eth) | Q1 Type (pos-Eth) | Maximum Current through Load |
|---|---|---|
| BC 557 | BC 547 | LED/Small Lamp (0.1A/100mA) |
| BC 461 | BFY 51 | Medium-sized Lamp (1A) |
| TIP 42A | TIP 41A | Big Lamp/Electic Heater(*) (6A) |
(*) Please note that although you can use an electric heater at this current limit, albeit a small one, the transistor will function extremely well as such a device itself, and thus requires a substantial heatsink. Be aware, however that the tab of the transistor will almost certainly be connected to the collector terminal, and making an electrical connection to the body of the vehicle could result in a short circuit which could damage the transistor and associated electronics.
I recommend a 1kΩ resistor for R1 at 12V, or 430Ω for 6V, but this alters linearly for other voltages.
