Strobe Alert System Circuit
Posted on Feb 1, 2013 11384
The circuit is activated by an LED/photoresistor isolator (Ul), which is a combination of a light-dependent resistor (LDR) and an LED in a single package. That device was chosen because of its high isolation (2000 V) characteristic, which is necessary because the strobe part of the circuit is directly connected to the ac line.The voltage divider is formed by R2, Ul`s internal resistance, and R3. When Ul`s internal LED is off, Ul`s internal LDR has a very high resistanceon the order of 10 . The voltage applied to NE1 is considerably below its ignition voltage of approximately 90 Vdc.The optoisolator`s internal LED is activated by a dc signal supplying 20 mA. The external sensor (s) that supply the signal are connected to the strobe part of the circuit at,11 and J2.
When the internal LED lights, the LDR`s resistance decreases to around 5 kQ. Under that condition, about 125 Vdc is applied across CI, R4, and C2. The neon lamp periodically fires arid extinguishes as capacitor C3 charges through R4, and discharges via NE1 and the SCR gate.Resistor R4 restricts the current input to C3, and thereby controls the firing rate of NE1about three times per second. The discharge through NE1 is applied to the gate of SCR1.SCR1, a sensitive-gate unit, snaps on immediately when NE1 conducts, which completes ..the ground circuit for transformer T1 (a 4-kV trigger transformer). As SCR1 toggles on and off in time with the firing of NE1, capacitor C2 (connected in parallel with Tl`s primary) charges via Rl, and then discharges very rapidly through Tl`s primary winding. A voltage pulse is applied to the trigger input of FL1, a Xenon flash lamp.It is important to remember that the circuit is connected directly to the ac line. Resistor R6 is included to limit the amount of line current available to the circuit. The value of R6 can be decreased if you intend to modify the circuit for more flash power.Warning: Even though the circuit is fuse-protected, it can still be dangerous if handled carelessly.