Conrad Course material 3964 14 years and over 3964 Manual De Usuario
Los códigos de productos
3964
10
Fig. 25: Stabilisation of LED brightness (Aufbau7.jpg)
Check the results with a new battery and a heavily used one. As long as a certain residual voltage is present, the LED
stays almost as bright. With only one LED, the battery voltage may be lower than with two LEDs, in which latter case
at least about 6 V still have to be present.
9.
Step: Temperature sensor
This circuit shows temperature differences via the LED brightness. It is enough to touch the temperature sensor with
your finger. The circuit depicted in Fig. 26 shows a so-called current mirror. The current through the 1 k
your finger. The circuit depicted in Fig. 26 shows a so-called current mirror. The current through the 1 k
Ω resistor is
mirrored in the two transistors and reappears in nearly the same magnitude as the collector current of the right
transistor. Since in the case of the left transistor base and emitter being interconnected, a base-emitter voltage of
about 0.6 V automatically appears which leads to the specified collector current. Theoretically, the second transistor
should now show the same collector current with exactly the same data and with the same base-emitter voltage. In
practice, however, usually slight differences result. The current mirror is at the same time a constant current source.
The brightness of the yellow LED thus doesn’t change if you bypass the green LED.
transistor. Since in the case of the left transistor base and emitter being interconnected, a base-emitter voltage of
about 0.6 V automatically appears which leads to the specified collector current. Theoretically, the second transistor
should now show the same collector current with exactly the same data and with the same base-emitter voltage. In
practice, however, usually slight differences result. The current mirror is at the same time a constant current source.
The brightness of the yellow LED thus doesn’t change if you bypass the green LED.
Fig. 26: The current mirror (Schaltung8.jpg)
Fig. 27: Transistor as temperature sensor (Aufbau8.jpg)
The circuit is suitable as a sensitive temperature sensor. Touch one of the transistors with your finger. The heating
that appears changes the output current and becomes visible in the LED’s change in brightness. Depending on which
of the two transistors you touch, you can increase or decrease the brightness of the right LEDs somewhat. Depending
on ambient temperature, you can heat it up by up to 10 °C with your finger and which already becomes quite visible.
The difference in brightness becomes even clearer if you carefully heat one of the transistors with a soldering rod.
red
green
yellow