A useful timing strobe can be constructed using high-brightness LEDs and
a few common components. Ignition pulses from the number 1 cylinder
high-tension lead are used to trigger the circuit via a home-made
inductive pickup. Transistors Q1 & Q2 buffer and amplify the pulses
from the pickup, which then drive the inputs of three Schmitt-trigger
inverters (IC1a, IC1c & IC1f). Each positive pulse at the inverter
inputs causes a low pulse at their outputs, forward-biasing D2 and
immediately discharging the 6.8nF capacitor. When the capacitor is
discharged, the inputs of the second bank of three inverters (IC1b, IC1d
& IC1e) see a logic low level, so their outputs go high, driving Q3
into conduction and powering the LED array. After the pulse ends, the
IC1a, IC1c & IC1f inverter outputs return high, reverse biasing D2
However, it takes some time for the 6.8nF capacitor to charge to the
logic high threshold voltage of the inverters’ inputs, effectively
stretching the initial pulse width and lighting the LEDs for the
required amount of time. The pickup can be salvaged from an old Xenon
timing light or made up from a "C" type ferrite or powered iron core
large enough to fit around a HT lead. Some experimentation will be
required to determine the number of turns required to achieve reliable
triggering. About 100 turns of light-gauge wire proved sufficient on the
prototype. A cleat is used to close the magnetic path around the lead
and is held in place with a large battery clip. Miniature screened
microphone cable can be used to connect the pickup to the circuit, to
prevent interference from other sources.
Miniature screened microphone cable can be used to connect the pickup to the circuit, to prevent interference from other sources. LedConceptsLighting
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