Flickering Light II


Posted on Feb 6, 2014

Regardless of whether you want to effectively imitate a house fire, a campfire, or light from welding, the circuit described here fills the bill without using a microcontroller, although it does use a larger number of components (including some truly uncommon ones). The circuit is based on three oscillators, which are built using unijunction trans


Flickering Light II
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istors (UJTs). Each oscillator has a different frequency. The output voltages are mixed, which produces the flickering effect. A unijunction transistor consists of an n-type bar of silicon between two ohmic (non-barrier) base contacts (B1 and B2). The effective resistance is controlled by the p-type emitter region. The designation transistor` is a somewhat unfortunate choice, since it cannot be used for linear amplification. UJTs are suitable for use as pulse generators, monostable multivibrators, trigger elements and pulse-width modulators. If a positive voltage is applied to the emitter (E), the capacitor charges via the resistor. As soon as the voltage on the emitter reaches approximately half the supply voltage (for a 2N2645, the value lies in the range of 56 75 %), the UJT fires` and the capacitor discharges via base B1 and the resistor, generating a positive pulse. The UJT then returns to the non-conduct state, and the process just described repeats periodically. The frequency can be approximately given by the formula f 1/(RC) The frequency is independent of the value of the supply voltage (which must not exceed 35 V). The maximum emitter blocking voltage is 30 V, and the maximum permissible emitter current is 50 mA. The values of resistors R1, R2 and R3 can lie between 3 k and 500 k. If necessary, the frequency can be varied over a range of 100:1 by using a trimpot instead of a fixed resistor. The frequencies from...




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