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Through two LEDs, one green and one red indicator
informs that a level of
low battery voltage. When the voltage drops below
mounting a certain
threshold (adjustable via a potentiometer, for example
threshold to 21.6 V), the red LED lights, and
The green LED turns off. The voltage from the 24V
battery is applied to a resistive divider bridge formed
resistor R1 and potentiometer RV1, whose divisive
visited
adjustable with RV1. This knob to set the threshold
below which
lighting the red LED occurs. One finds the cursor of
the latter, a
voltage whose value depends on its position, for
example 7.89V when the voltage of the
battery is 24V and the slider is in a central position
RV1. Under these conditions, the
Zener diode D1 which reverse conduction threshold is
7.2V, and the anode of which is connected
to ground via resistor R4, is conducting, and
produces a voltage drop of 7.2 volts
its terminals. Thus found on the base of transistor
Q1, a voltage value of
sufficient to be driven. The absence of a current
limiting resistor between
anode of D1 and Q1 base you may be startled, but not
to worry,
the current is limited due to the presence of the
resistor R1 in series with the potentiometer
RV1: even when the cursor RV1 is "up" position
(toward + V), no risk. the
transistor Q1 being turned on, it allows the passage
of a sufficient current between the emitter
and its collector, resulting in the lighting of the green
LED D2. The voltage at
the emitter of the transistor Q1 is however
insufficient to saturate the transistor Q2,
which thus remains blocked, and D3 red LED remains
off. If now the voltage of the
battery drops, the voltage at the slider of VR1 fall
simultaneously. If the voltage
RV1 the cursor becomes lower than the voltage of the
reverse conduction diode
Zener D1, that is to say if the voltage drops below
7.2V, the base of transistor Q1 is
receives more than enough power to saturate, and it
crashes. Result: more
sufficient current between emitter and collector of
Q1, D2 green LED goes out. but
Meanwhile, the green LED D2, even if it is off, always
allows passage of a
enough current to power the base of transistor Q2,
which sees this as a good opportunity to
talking to him: it is saturated and ignites D3 red LED.
It is possible to obtain an effect to transition between
a more or less gradual extinction
and the green LED switches on the red LEDs, playing
on the value of this resistor R5
(between 20 ohms and 20 ohms). If you want the
green LED remains lit even
when the red LED lights up, you simply replace the
resistor R5 20K by
resistance of 20 ohms. Do not worry, the base current
of Q2 will not become
dangerous, no risk of destruction for the transistor. If
all this intrigue is
you are about to do an exercise interesting
research;-).
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