CURRENT ISSUE
Contests
Test your eq
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Issue #232 November 2009
Problem 1—While reverse-engineering an LCD module with LED backlight, a colleague came across this circuit and had trouble figuring it out. Let’s see if we can help him out.
Point A connects to one of the external pins of the module and appears to be used for switching the backlight on and off.
First of all, what is the purpose of R4?
Problem 2—What is the purpose of R1, R2, and Q1?
Problem 3 —What can you say about the LED current with respect to the actual voltage on point A?
Problem 4 —What can you say about the LED current with respect to ambient temperature?
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