CATV Upstream Fiber-Optic Receiver

Upstream fiber links in a community antenna television (CATV) system are usually among the most difficult elements of the network to align properly. 

Set-top boxes and cable modems employ "long-loop" automatic gain control (AGC) (in other words, far-end power-control), which attempts to normalize the RF path end-to-end.

Alignment is very important because for every decibel of uncorrected optical loss, there's a 2-dB optical-modulation-index (OMI) variance at the transmitting node. 

This can easily lead to intermodulation problems when multiple services depend on simultaneous use of the link.

Dual zone alarm schematic

This brilliant circuit shows a two-zone alarm - with automatic exit, entry and siren cut-off timers.

It can be triggered by the usual types of normally-closed input devices - such as magnetic reed contacts - foil tape - PIRs etc.

The circuit will work at anything from 9 to 15-volts. All you need do is select a siren, buzzer and relay to suit the voltage you want to use.

Car Battery Charger

This circuit shows car battery charger that will easily charge most any lead acid battery. The charger delivers full current until the current drawn by the battery falls to 150 mA. 

At this time, a lower voltage is applied to finish off and keep from over charging. When the battery is fully charged, the circuit switches off and lights a LED, telling you that the cycle has finished.

Plus and Minus DC Power Supply

This is a classic example of a regulated DC power supply that produces both a positive 15v and a negative 15v from a 20vac wall adapter.

Phone ringtone generator circuit

This is a simple home telephone ringtone generator circuit which is built with applying only several electronic components / parts. 

It generates simulated telephone ringtone and requires only DC supply with 4.5V DC to 12V DC voltage.

One may possibly use this circuit in ordinary intercom or phone-type intercom. 

The sound is pretty loud when this circuit is operated on +12V DC power supply. Even so, the volume of ring sound can be adjusted.

Rain Detector schematic

Here's a simple rain detector circuit. It uses a sensor made of a small piece of etched PC board and a simple SCR circuit to detect rain and sound a buzzer. The SCR could also be used to activate a relay, turn on a lamp, or send a signal to a security system.