This touch-plate doorbell makes use of enhancement-mode MOSFETs forming part of CMOS quad NAND gate CD4007B in conjunction with a detector and Darlington driver stage.
IC CD4007B comprises a dual complementary pair and an inverter. It has three p-channel and three n-channel enhancement-mode MOSFETs as shown in Fig. 1. The devices can be individually accessed for use as very high-input op-amps besides their use as linear amplifiers, current sinks/sources and crystal oscillators.
The circuit of the touch-plate doorbell is shown in Fig. 2. Here, we have used the complementary pair comprising common gate (pin 10), individual drains (pins 9 and 11) and common output (pin 12) as a very high-input micropower linear amplifier capable of amplifying the hand/finger induced AC voltages by a factor of ‘40.’ The output of this complementary stage is further amplified by a single-stage n-channel MOSFET (gate=pin 6, source=pin 7 and drain=pin 8). The output of this stage is detected by the detector circuit comprising diodes D1 and D2, with R4 acting as a bleeder resistance across C2. The detected voltage is amplified by the Darlington stage comprising transistors T1 and T2, which drive an LED-buzzer combination of LED1-PZ1 when a person touches the metallic sensor plate. The sensor plate can be suitably mounted on the entry door. For power supply, you can use either a 9V battery or a battery eliminator.