Solidstate Signal Lamp

By T.K. Hareendran


This solidstate signal lamp is a good alternative to revolving mechanical signaling lamps used in ambulances. It uses ultra-bright blue LEDs for signaling. However, you can replace blue LEDs with bright red LEDs for other applications.

Solidstate signal lamp circuit

Fig. 1 shows the circuit of the solidstate signal lamp, and its construction plan is shown in Fig. 2. Interconnect point A, +12V supply, and GND shown in Figs 1and 2.

solidstate signal lamp
Fig. 1: Circuit of solidstate signal lamp

Before the vehicle driver closes the front door, switch S2 is in normally opened (N/O) condition as per the mechanical arrangement. After closing the door, closing of switch S1 powers up the whole circuit via polarity-protection diode D. Gates N1 and N2 are immediately set in the bistable mode by R2 and C1 and the resulting high output at pin 3 of gate N2 enables the low-frequency oscillator wired around gate N3. Components C2, R3, and VR determine the oscillator frequency, which can be changed to some extent by varying VR.

Consequently, IC2 (CD4017) is clocked by gate N3 and the output of IC2 changes sequentially from Q0 (pin 3) to Q3 (pin 7). These outputs are used to switch four LED driver transistors T1 through T4 via resistors R4 through R7, respectively. The sets of three ultra-bright blue LEDs, each connected to the collectors of driver transistors T1 through T4, respectively, glow sequentially as the outputs Q0 to Q3 go high. The result is a revolving blue light effect, the same as that obtained from a mechanical signal lamp.

Fig. 2: Construction plan
Fig. 2: Construction plan

Circuit operation

Whenever the front door is opened, switch S2 gets activated (opens) and makes the bistable to immediately reset the output of gate N2. Thus the clock oscillator is disabled and the signaling function is stopped. For good visible effect, high-quality miniature reflectors should be used.

The article was first published in July 2003 and has recently been updated.


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