transistors
I was wondering if anyone can help me understand how this circuit works.
I understand covering the photo conductors increases it's resistance but I'm not sure how the transistors come into play. This is a bidirectional motor and it is activated when I cover one of the photo conductors
Thanks in advance!
The first comprehensive logic series was the TTL series 74xx. This used BJTs (Bipolar Junction Transistors). Later there came variants like the often used 74LSxx, where the "LS" stands for Low-power Schottky TTL. As the name implies these used less power than the rather power-hungry TTL, and were faster too. At the same time the CMOS 4000 series was developed. The "C" in CMOS stands for Complementary, meaning it's a combination of N-channel and P-channel MOSFETs. Their construction is simpler than TTL and they use far less power. Later standard CMOS developed into HCMOS, "H" for High-speed. Most 74LSxx types have been released as HCMOS in the 74HCxx series, or the 74HCTxx series, which is TTL compatible. Later more variants were developed, like Advanced CMOS (74ACxx).
Microcontrollers are built in HCMOS technology, so they use MOSFETs. AFAIK JFETs aren't used for logic ICs. The transistor you show in the picture is a BJT, which you can tell from the pin designation:
E = Emitter
B = Base
C = Collector
For a MOSFET the pins would be
S = Source
G = Gate
D = Drain
respectively.
Many ICs in the 74HCxx series were originally released in 14 or 16 pin DIL packages, which meant that they would fit four 2-input gates. With miniaturization (SMT) came the demand for smaller packages, even if they contained less gates. Several manufacturers offer single-gate and dual-gate versions of logic gates. For example, NXP has a 74LVC1G00 (single 2-input NAND) and a 74LVC2G00 (dual 2-input NAND) version of the classical 74HC00. 74LVCxx is yet another HCMOS technology. This page lists all NXP logic families.
Found a great animation here:
http://www.falstad.com/circuit/
(then choose from the menu: circuits/transistors/multivibrators/astable multivib
Best Answer
The reason for the transistors is that the photo-resistors can't provide enough current to drive the motor. The transistors act as an amplifier of the small signal from the photo-resistors to make the signal large enough to power the motor directly.
These are darlington transistors, which will start to turn on when the voltage between the base (connected to the the photo-resistors) and the emitter (connected to the motor) reaches about 1.4V. They have very high gain, so only a very small current at the base is required to give a large current flowing from the collector to the emitter.
when both photo-resistors are at the same light level, the voltage between them is close to the middle the +5(VCC) and -5V(VEE) rails (zero volts). The emitters (connected together and connected to the motor) are also at zero volts, as they are connected through the motor to the ground (0V). In this state, there is no voltage between base and emitter to begin to turn on either of the transistors.
When you cover one of the photo-resistors (say R2), the voltage at the junction of the two resistors moves towards the other rail (so if you cover R2, it's resistance increases, and the voltage at the junction between R2 and R3 moves towards the -5V rail.
Once this voltage moves below -1.4V, Q2 begins to turn on. Only a very small current at the base of Q2 is required to have the emitter of Q2 draw a large current from the collector. The transistor will draw as much current as needed from the collector to keep the voltage at the emitter 1.4V above the base, so as you reduce the light at R2, you lower the voltage at the base of Q2 which results in Q2 drawing more current from the -5V rail to keep the emitter at 1.4V above the base. This results in the motor turning faster.
Exactly the reverse happens when you cover R3. Now the voltage at the base of Q3 (and Q2) increases, and Q3 begins to draw current from the +5V rail to turn the motor in the opposite direction.