T A Babu
EDN
The key features of this Design Idea are:
- Soft switch on/off operation
- Zero current drain in sleep mode
- Two level dimming
- Low battery protection
- Simple controller based on CMOS CD4017 Johnson counter
The circuit is divided into two portions for easy understanding: Logic control (Figure 1) and LED driver (Figure 2).
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| Figure 1. | LED controller. | |
The circuit is controlled by a momentary push switch. Until SW1 is pressed, the whole circuit is in sleep mode, drawing zero current. Closing SW1 allows Q2 to conduct with the aid of IC1, a TL431 programmable zener, which in turn powers IC2 and MOSFET Q3.
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| Figure 2. | Controllable LED boost converter. | |
At this time, the Q0 output from pin 3 sets the LED brightness to 25% as determined by R14 (Figure 2). Each push feeds a clock pulse to IC2.14, which increments the count. The second push makes Q1 go high, with R15 now setting the second dimming level to 50% brightness. R14 & R15 inject a current into IC3’s feedback node to achieve the various dimming levels. The third push sets IC2’s unconnected output Q2 high (leaving Q1 & Q2 low) to set the LEDs to 100% brightness. The fourth push sets Q3 high, pulling down the bias of IC1 and allowing the whole circuit to enter sleep mode.
When the battery voltage drops to around 11.2 V, IC1 stops functioning, as set by R4, R5, the VCE drop of Q2, and the forward drop of D4 (in normal operation, the TL431 keeps the low battery indicator (LED1) off with the aid of D2). To prevent further low-battery rundown, LED1 glows for a brief moment, then goes dark, but glows continuously as long as SW1 is pressed.
A typical MC34063 DC-DC converter serves here as a LED driver, which is a cost effective and easily available solution. Inductor L1 can be a self-made or, e.g., a Würth part (744743221). Open load protection is handled by D10. If necessary, adjust R14 and R15 to get required brightness levels.
This design may be applied in LED lanterns, reading lights, camp lights, and general purpose lighting. The battery can be charged by solar, AC, or car adapters, or any other suitable source.
