Simple two-transistor circuit lights LEDs

Simple two-transistor circuit lights LEDs

Barry Tigner @ has a design idea on how to power a LED from a 1.5V battery using two easily available transistors.

A previous Design Idea describes a circuit that uses an astable multivibrator to drive an LED (Reference 1). The circuit in Figure 1 uses a simpler alternative approach. The circuit uses a 2N3904 NPN transistor and a 2N3906 PNP transistor, which operate as a high-gain amplifier.

Simple two-transistor circuit lights LEDs – [Link]

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That was the standard circuit that Chinese toys used to power an LED about 30-40 years ago. Coin cells were expensive then, so this made sense. Nowadays, lithium coin cells have become so cheap, and limit the current automatically, that everybody just uses a single cell for a red/green/yellow LED, and two for a blue/white one.

There was a variant of this circuit that could blink the LED by using just one additional capacitor, but I don’t remember it exactly.


Nothing new here. This is just one of the possible ‘Joule thief’ variants – basically a self-oscillating boost circuit, which can drive LEDs from as low as 0.7V or so. Such circuits have been used in solar garden lights, etc for many years / decades.

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