A solid-state bell push is far more robust than the usual mechanical type. The touch contact may be made from an audio socket, which has a high insulation resistance and is practically indestructible.
The inverting (-) input of IC1 is at half the supply voltage via R4-R5, while the non-inverting input is at earth potential via R3. When the contact is touched (less than 10 MΩ, a light touch is fine), the output of the opamp goes high and the relay (a 9-V or 12-V type) is energized. The relay contacts then actuate the (existing) bell. Resistor R1 and capacitor C1 ensure that the e-bell cannot be actuated accidentally.
The circuit is powered by the rectified bell transformer voltage (or a second bell transformer) for safety’s sake, do not use a different type of transformer). A bell transformer can normally provide a current of 1 A, so the first solution is almost always possible.
The circuit draws a (quiescent) current of only 5 mA. If a 741 is used and only 0.5 mA if a TLC271 is used. When the relay is energized, this rises by 30 mA.
The TLC271 operational amplifier combines a wide range of input offset voltage grades with low offset voltage drift and high input impedance. In addition, the TLC271 offers a bias-select mode that allows the user to select the best combination of power dissipation and ac performance for a particular application. These devices use Texas Instruments silicon-gate LinCMOS technology, which provides offset voltage stability far exceeding the stability available with conventional metal-gate
With the programmability options of the TLC271, a designer can choose a very low current option allowing for extended battery life, or choose a higher current option for more performance. It is possible to switch performance modes as the application demands change. The TLC271 is well-suited for many consumer audios, industrial, and other low-power applications.
Filtering – Equalizers
Portable Meters and Measurement