Bulb Failure Warning circuit

Auto 12V Bulb Failure Warning!

Electronic circuits and projects are becoming more and more complex, which makes it increasingly difficult for a novice/student to get up to speed. Here, we therefore aim to get back to the first harmonic, and present an interesting little project with generally available discrete components. This is a very basic warning light designed to let you know when a bulb, for example the rear light, in your automobile fails. This minuscule circuit can be used in almost all automobiles with a 12 volt dc electric supply system.

Bulb Failure Warning circuit

In the circuit, a type TLP521-1 opto coupler (PC1) monitors the status of the connected light bulb. When the lamp is switched on in the automobile, a current flows through the light bulb via the 1N5401 diodes (D1&D2). This causes the light emitting diode in the opto coupler to lights up, and as a result the opto coupler deactivates the pnp transistor BC559 (T1). In case of a bulb failure, the opto coupler is turned off instantly to activate the 5mm- red warning light (LED1) via T1. Resistor R3 limits the operating current of LED1, and resistor R2 (100K- test selected) determines the switching threshold of T1 to some extent.

The TLP521-1 consists of a photo-transistor optically coupled to a gallium arsenide infrared light emitting diode in a 4- pin package. The TLP521-1 has a minimum current transfer ratio (CTR) of 50 %, and is designed for applications requiring good galvanic isolation. The 1N5401 is a general purpose 3.0A plastic rectifier for use in rectification of power supplies, inverters, converters and freewheeling diodes application. Although this circuit can be built without an opto-coupler, we prefer to add such a component for design flexibility.

enclosure suggestion

(enclosure suggestion)

While some dexterity is needed, the circuit can be built in fairly easily. Try to use a small piece of perforated circuit board and then fitted the ‘encased’ unit (see the enclosure suggestion) somewhere close to the dash board. Our prototype has been tested extensively on various vehicles, and it works faultlessly.


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  • T.K.Hareendran

    Istvan: There is always some drift between theory & Practice. That’s why I really build my prototypes (instead of simulations) since the last 25 years. If your circuit is working well in real-world it’s nice!

    • Istvan

      T.K.Hareendran: I think it works well. It is just a design for fun not for real needs. I bet it works. 🙂

  • Istvan

    Please look at the attached 2 circuits. The left is in case of bulp failure, the right is in case of bulp OK operation. I hope with this help you understand the operation of this circuit.

    Calculating current source’s current:


    Looks interesting! I remember another circuit where the feed to the lamps was wound around a reed relay in such a way as to balance the magnetic field if both bulbs are okay. If one failed then the unbalance would magnetize the relay and then the warning is relayed to the driver a lamp or hooter or whatever! This was to monitor brake lamps which would look then as a turn signal in cars with flashing red lights and the same bulb also used as brake light

    • T.K.Hareendran

      @KROKKENOSTER: It’s another good idea I think. Thanks for your valuable feedback!

  • T.K.Hareendran

    Istvan: Please give a brief description (esp R1,D3,D4…) of your design,and point out what are the advantages you prefer to highlight!

  • Istvan

    I think my design is cheaper. Please look at mine design.