Data Sheet
129. Load on Emitter
Build the circuit shown on the left and turn on the switch (62). Whenever light shines on
the photoresistor (68), the lamp (76) will be off. Cover the photoresistor (68) with your
nger and the lamp (76) will turn on. This circuit demonstrates that the photoresistor (68)
and NPN transistor (50) can be used like a switch to turn on and off a load (in this case
the lamp (76)), and the load can be placed on either the Emitter of the NPN transistor
(50) (like in this project) or the Collector of the NPN transistor (50) (like in the previous
several projects). The main difference is that when you place the load on the Emitter, then
the internal resistance of the load will increase the voltage level that is required at the
Base to turn on the ow of current from the Collector to the Emitter.
130. RC Circuit
Build the circuit shown on the left and turn on the switch (62). Press the press switch
(61) and you will see the colorful LED (72) light the ber tree (40). Release the press
switch (61) and the colorful LED (72) will stay bright briey and then turn dim. This is
an example of an Resistor-Capacitor (RC) circuit. This is a rst order RC circuit because
there is a single resistor and single capacitor in the circuit.
131. Resistor in RC Circuit
Replace the 1kW resistor (42) in project #130 with the 5.1kW resistor (43) and turn
on the switch (62). Press the press switch (61) and you will see the colorful LED (72)
light the ber tree (40). Release the press switch (61) and the colorful LED (72) will stay
bright for a little while and then turn dim. This RC circuit has a larger 5.1kW resistor
(43) compared to project #130, which limits the current and thus discharges the 100mF
capacitor (73) more slowly.
132. Delayed Lights
Replace the 1kW resistor (42) in project #130 with the 10kW resistor (44) and then turn
on the switch (62). Press the press switch (61) and you will see the colorful LED (72) light
the ber tree (40). Release the press switch (61) and the colorful LED (72) will stay bright
for a while and then turn dim. This circuit could be used in your house to keep the lights on
for a short while after you turn them off so that you can exit the room before it gets dark.
133. RC Time Constant
Replace the 1kW resistor (42) in project #130 with the 100kW resistor (45) and then turn
on the switch (62). Press the press switch (61) and you will see the colorful LED (72) light
the ber tree (40). Release the press switch (61) and the colorful LED (72) will stay bright
for many seconds before turning dim. Through use of Kirchhoff’s Current Law, the voltage
level of the 100mF capacitor (73) as a function of time can be determined. Solving this
requires differential equations which is beyond scope for this manual, but the result is that
the voltage of the capacitor decays as an exponential function with a time constant of R*C.
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