Soft-Action Mute Switch

Description:

This simple circuit is a line-level audio signal muting switch based on a soft-action power on/off process. When the S1 switch is closed, R4, C1 and Q4 JFET quietly ground the signal to be muted in between 100 to 200 ms and the RED indicator light goes on. When the S1 is opened the signal is released and the GREEN led goes on.

Potentiometer R2 must be set to the value twice of the transistor's cut-off voltage so the on/off transition times become approximately equal. R2 and D3 discharge C1 swiftly and mute the signal during the power down. In this process, the signal path should remain stable to below 1/3 of the normal supply voltage (In this example below +/-4). Then Q1 finishes muting.

2.4 GHz RF Pinhole Camera Amplifiers

Description: 

Here are two simple Junk Box but extremely useful circuits. They are designed to amplify those flimsy 2.4 GHz Pinhole cameras with such annoyingly low power and short range.

The two transistor version [A] is merely a cascaded version of [B] for greater output. The one transistor version in [B] will easily penetrate two walls of an average household, while the cascaded version [A], will penetrate two floors. Needless to say, a pinhole RF camera’s Antenna connection connects to the Input of either amp. Either amplifier’s ground [Negative] connects to the camera’s Negative, and off course, Positive to Positive. The amps should use the camera’s DC voltage if possible, especially if running the camera off a DC plug pack. All component leads and connections should be short as possible. [point to point connections]. Antenna connects to the RF output.

Note: All inductors are molded chokes. They are critical. Other Higher Gain transistors could be used, but values would have to be altered appropriately.
For those intending to build either project, I strongly recommend you begin with the one transistor version for two reasons. It’s simpler and cheaper, and, when using surface mount components, will easily solder directly on the back of the camera’s circuit board.

When built with care and success, results will be excellent, and will not only encourage you to build the higher power version, but you will have acquired the skill to do so successfully. Most 9 Volt RF Cameras will work quite safely on 12Volts.
 
Caution: You will be broadcasting Video and probably Audio. The two transistor version is capable of transmitting around ½ Kilometer with the camera’s original antenna, and a great deal further with an extended antenna. It makes Public Listening and Viewing!. As far as I’m aware, the power outputs far exceed the Legal Limits set for such devices in most countries, so, if you live in a built up area, please keep this in mind.

Designer & Author: Special thanks to Laszlo Kirschner.

Series Protection Against High Voltage Transients

Description: 

In many low-voltage applications, undesired voltage overshoots may occur at the input during the circuit operation. To avoid the harm of this overvoltage, we generally use parallel clamps such as fuses or other current limiting devices. The disadvantage of the parallel protection may be resetting or renewing the protector component. This circuit provides a series protection  that turns off the power switch using a series connected high voltage MOSFET (n-channel – Q1) power switch and a fast overvoltage detector IC, MAX1614. The series connected  rectifier diode D1 and the power switch is together capable of protecting the load up to +/- 500V of either polarity.