The Silicon Controlled Rectifier

The basic four-layer diode has a number of useful properties and capabilities, but would become even more useful if it could be more accurately controlled. To do this, however, we must gain access to more than just the outer ends of the device.
The four-layer construction shown to the right is known as a Silicon Controlled Rectifier, or SCR. To form it, we have added a connection to the p-type region next to the cathode. This connection is known as the gate.
If we ground both the cathode and the gate, and apply a positive voltage to the anode, no current will flow through this device. This is in keeping with the basic four-layer diode. In this case, however, we will not allow the applied anode voltage to exceed the SCR breakover voltage. Thus, if nothing happens, the SCR will remain turned off indefinitely.
However, if we now apply a small positive voltage to the gate lead sufficiently to forward bias the cathode junction, the device will immediately turn fully on. Again, this is in keeping with the behavior of the basic four-layer diode. The difference is that we can accurately control the timing and the applied gate voltage, if necessary. Thus, we can determine the conditions under which the SCR will fire more accurately than we can for the basic four-layer diode.
A modern application for the SCR is the "cyclops" brake light on all cars now sold in the USA. This circuit, as shown in the schematic diagram to the right, uses two SCRs cross-connected with each other (each gate connected to the other SCR's anode) and triggered from the regular brake lights on either side of the rear of the vehicle. The lights are connected to the two cathodes, which are connected together so that either SCR can keep both lights on once they are triggered. If only one SCR is triggered (as when you use a turn signal), the triggered SCR gets no anode voltage from the opposite brake light, so the "cyclops" light remains off. Only if both brake lights are on together will the "cyclops" turn on. Once it does, it remains on regardless of turn signals as long as the brake is applied. When you release the brake, power is removed from the "cyclops" as well as from the brake lights and they all turn off.
The two resistors have a relatively high resistance, and are not critical in any case. They ensure that the gates of the two SCRs are held off while the brake lights are unpowered. The resulting circuit is simple and inexpensive, yet quite robust and easily able to handle the bumps and jolts of an automotive environment.
Another practical application is in a telephone "hold" circuit. Triggering the SCR causes the circuit to contiuously draw current through the telephone wires, thus causing the switching station to assume a phone is still in use. You must hang up the phone while still pressing the button in order to ensure that the SCR remains triggered. Picking up the same or another receiver in the house reduces the current through the SCR enough that it turns off.
You can use multiple copies of this circuit. However, only the LED on the active "hold" circuit will light up. Also, pressing the button while no phone is in use will engage that "hold" circuit at once, so it should be used with care. And of course you should check with your local phone company before connecting anything to the phone line. This is to protect your circuitry as much as to protect the phone company from improper and possibly damaging connections.