Simple Touch Pad Dimmer

 

Using a Power-FET it is possible to build a very simple touch dimmer for low voltage lamps. Two drawing pins are used here as the touch contacts. The electrical resistance of your skin is in the order of 100 kQ to 1 MQ. The circuit operates as an integrator with a capacitor in its negative feedback path. This configuration gives a relatively linear control character¬ istic. Once you have selected a brightness level, it will be maintained for hours if you use a low leakage (foil) capacitor. Another feature of this circuit is that the harder you press on the contacts, the quicker the lamp brightness changes.

Low-Noise Microphone Amplifier

The signal from a microphone is two weak for a standard line input. This low-noise DC-coupled microphone amplifier pro¬ vides a solution for anyone who wants to connect a micro¬ phone to his or her hi-fi installation. As can be seen from the schematic diagram, a good circuit does not have to be com¬ plex. A differential amplifier is built around T1 (MAT-03E), which is a low-noise dual transistor. The combination of T2 and LED D1 forms a constant-current source for the input stage. A low-noise opamp (OP-270E) amplifies the difference signal that appears at the collectors of the dual transistor. The result is an analogue signal at line level. The bandwidth of the amplifier ranges from 1 Hz to 20 kHz. Within the audio range (20 Hz to 20 kHz), the distortion is less than 0.005 percent. Since only half of the OP-270E is used, the remaining opamp could be used in the output stage of a stereo version.

The amplifier can be powered from a stabilised, symmetrical supply with a voltage between ±12 V and ±15 V. Such sup¬ ply voltages are already present in many amplifiers.