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11-04-2015

LED strobe has independent delay and duration

NE555

Michael Page

EDN

The circuit in Figure 1 is not complex, but it saved the day in an application involving visual inspection of the spray pattern of fuel injectors for quality and consistency. In this application, xenon strobe lights did not work because they take up too much space, and the light they emit is too intense. With a bank of six injectors with isolation panels, the reflection off a person’s shirt or the wall behind him would interfere with the visual inspection. So the application instead used white HB LEDs (high-brightness light-emitting diodes) on “gooseneck”-type stands for adjustability in the chamber sections. Although the application could have used the trusty old 555 timer, the delay and duration duty-cycle controls interact, which is an awkward situation.

LED strobe has independent delay and duration
Figure 1. This circuit employs HB LEDs for a visual-inspection application.

The circuit in Figure 1 shows the main-clock input; the delay and duration potentiometers, P1 and P2; and the HB-LED output. The circuit also includes an onboard general-purpose LED for bench testing to indicate an input signal, although, when the circuit is operating at high speeds, this LED is useless. The main-clock input is a 5 V pulse of approximately 30 µsec coming from the fuel-pump index. Delay potentiometer P1 adjusts the on-time delay of the LED from about 40 µsec to 2 msec, and duration potentiometer P2 adjusts the LED-on, or flash, time with a range of approximately 15 µsec to 15 msec.

LED strobe has independent delay and duration
Figure 2. With a main-clock input of 650 Hz, the delay is approximately 250 msec, with P1 at 10%,
and the duration is approximately 600 msec, with P2 at 75%. The top trace (blue) represents
the strobe delay, the lower trace (green) represents Q1’s base duration,
and the 5 V trace (red) represents the main clock.

The circuit applies a 5 V pulse, the main clock, to diode D1 and capacitor C1 to form a peak-hold circuit. C1 then discharges at a rate that P1 sets. Schmitt trigger IC1A monitors C1’s voltage, and, when it reaches the low threshold of IC1A, it outputs a high level to IC2’s clock input, setting the Q output high. With IC2’s Q output high, the Darlington-transistor pair comprising Q1 and Q2 turns on, driving the output to the HB LED low at the output, lighting the LED. At this time, capacitor C2 charges at a rate that P2 sets. When this voltage reaches the upper threshold of IC1B, IC1C’s output switches to high, resetting flip-flop IC2’s output back to low and turning off the HB LED. The circuit is now ready for another round. Diode D2 ensures a complete discharge of capacitor C2 for repeatability when you reset the Q output of IC2 to low. Because IC2 requires an active-high signal, you can omit IC1B and IC1C, but you should use a Schmitt trigger following an RC circuit for repeatability, especially on slow capacitor-charge/discharge times.

LED strobe has independent delay and duration
Figure 3. An adjustment change of delay occurs with the same duration as in Figure 2.
The top trace (blue) represents the strobe delay, the lower trace (green) represents
Q1’s base duration, and the 5 V trace (red) represents the main clock.

Figure 2 shows the results of the circuit running with a main-clock input of 650 Hz and a delay of approximately 250 µsec, with P1 at 10%, and a duration of approximately 600 µsec, with P2 at 75%. Figure 3 shows an adjusted change of delay with the same duration setting as in Figure 2. The new flash period overlaps the following fluid burst. You could, depending on the injector nozzle, see the end of one fuel burst of calibration fluid and the start of another in the chamber during the same flash period without encountering an error. The circuit also has a boost switch for a momentary intensity increase; otherwise, R3 normally limits the current to approximately 40 mA. When you press the boost switch, the Darlington pair, two 2N2222 transistors with current of approximately 400 mA, still limits the current, but long-term use of the switch will shorten the LEDs’ life. You should tailor the values of C1, C2, P1, and P2 to the application. Calculations will vary depending on the logic family you use, but, generally, T=0.7×R×C, where T is the time in seconds, R is the resistance, and C is the capacitance.

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