A transistor switch, is useful for switching heavier loads, than an output pin can source, it maybe in the form of a pulse to trigger a device, or pulsed output in burst cycles, or just on for a given length of time, and an easy way is to use a transistor as a switch. This method is applicable to devices which use relatively low voltage and direct current (dc). The current available at the collector of a transistor depends on how much it can conduct when it is switched on. Low power types usually allow a couple of 100mA, looking at the first diagram
what passes through the load passes through the transistor, if the load only requires a small current then a resistor may need inserting in series with the load. And also drop the remaining voltage if it was, say an led. If more current is required for a device, simply substitute a larger transistor with a higher power rating.
The second diagram shows that the load is powered when the transistor is off (low input)
and when the transistor is on most of the current is flowing through the transistor, thus by-passing the load. There is still a small amount of current flowing through the load, the effective resistance of the switched on transistor is much less than the resistance of the load. For example if the load is a lamp of some type this may not be enough to make it glow, but with other types of load even this small amount may be not acceptable, so an inverted version of the first diagram, through maybe a logic gate would be the solution, or as the third diagram shows;
a two stage transistor switch, this then overcomes this effective resistance problem as it still operates like the second diagram : High input load switched off. Diagram three (below) shows a reverse bias diode which can be simply a rectifier type if switching an inductive load, devices like motors relays coils etc, as when the power is abruptly cut off to these type of devices a back EMF voltage flows and usually 10’s or 100’s of magnitudes higher depending on certain factors. This diode clamps any spikes or transients which would damage the transistor. The other diagrams show how the transistor can be coupled up to a larger voltage rail say 12v, when maybe 5v or TTL is at its base. They also show depending on whether the base of the transistor is on or off, how the load is switched on or off, all these diagrams show the base switched on (high input) and what happens to the load. so the opposite (low input) would have the opposite effect.