This brick contains a PUT (Programmable Unijunction Transistor). The special thing about this component is the threshold voltage between anode and cathode that can be adjusted via a gate contact. If a voltage is applied in the forward direction, the resistance between the anode and cathode is very high until the threshold voltage is reached. If the gate is now contacted with a voltage positive to the cathode, the threshold voltage can be reduced, which causes the PUT to prematurely become low-resistance. Oscillator circuits can be easily constructed using a PUT.