Breadboards and circuit schematics are fine and good, but what is this circuit actually doing?
STAR’s initial design borrowed heavily from an open source circuit design by MightyOhm, with custom modifications added later to enable better control and integration with a Raspberry Pi.
This is a circuit to boost 3 V DC to 400 V DC using a 555 timer as an oscillator to drive an
inductor.
The 555 timer is connected in bistable mode, pulsing high (3 V) and low (0 V) in response to signals on the trigger (2) and reset (4) pins. The output pulse is delivered by pin 3.
When the output pulse (pin 3) is high (3 V), current flows into the base of transistor Q1, causing it to conduct. Current then flows from VCC through inductor L1. This current enters the base of transistor Q3, which connects pin 4 (reset) of the 555 directly to ground. This causes pin 3 (output) to go low (0 V), which switches off transistor Q1 and simultaneously feeds 0 V back into pin 2 (trigger).
Q1 switching off causes current to stop flowing through inductor L1. This sudden change of current flow creates a magnetic field that opposes the change, causing a back EMF to flow in the direction of diode D1.
That back EMF will have a voltage proportional to the rapidity of the current change.
V = -L dI/dtSuddenly, a 3 V pulse has become a high voltage pulse of around 400 V. The resulting 400V is then passed to the Geiger tube. Variable resistor VR1 permits adjustment of the current flowing through inductor L1, which consequently adjusts the output voltage.
The 0 V signal to pin 2 (trigger) triggers the output of pin 3 to go high again. In this way, the 555 timer oscillates between the trigger (2) and reset (4) pins, causing the output of pin 3 to go high (3 V) and low (0 V) as fast as it is able.
There are some interesting subtleties to this circuit. Typically, a 555 operating in bistable mode will not use pins 5 or 7. In the above schematic, we see that pin 7 (discharge) is also connected to the base of transistor Q1. What’s going on?
Pin 3 (output) and pin 7 (discharge) operate similarly, with one important distinction: when pin 3 goes high, pin 7 goes open (doesn’t conduct). When pin 3 goes low, pin 7 also goes low, but because it is connected in such a way that there is no capacitance between it and the base of Q1, it drops the base of Q1 to 0 V extremely rapidly, resulting in a very fast change in current.
Pin 5 (CV) is not used. If the 555 timer in question is a bipolar timer, it will require a filter
capacitor to ground to prevent spurious signals. If the 555 timer in question is a CMOS timer, it does not.
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