Serial DRAM non voltizer

Posted on Feb 6, 2014

This device contains a ringing generator and a tone oscillator. The tone oscillator is set to either 350 or 440 Hz. The ringing generator is a potted module delivering 86 Vac at 20 Hz. It is available from a source listed in the reference. (View) At the instant that power is applied to the 555 astable, timing capacitor C1 is initially discharged,

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causing the output of the chip output at pin 3 to be high. Once C1 has charged to about 2/3 of the supply voltage, its output goes low, and the discharge transistor turns on, draining the charge on C1. (View) The telephone audio interface-essentially, a simple isolation/couple circuit-isolates the phone line from any connected audio circuit without presenting any danger to the phone line, the equip-rnent, or the user. (View) Positive feedback is provided by resistor R4, which causes the latching. A positive pulse at the set input causes the output to go high and a reset positive pulse will return the output to es-sentially 0 Vdc. (View) The AD537 is well-suited for frequency-shift modulator and demodulator applications. Requir-ing little power, it is especially appropriate for using phone-line power. The Bell-System 202 data en-coder shown here delivers the mark frequency of 1. 2 kHz with the data input low. When the input goes high, the timing current increases to 165 A and generates the space frequency of 2. 2 kHz. The trim shown provides a ±10% range of frequency adjustment. The output goes to the required band-pass filter before transmission over a public telephone line. A complementary demodulator is easy to implement. (View) This circuit is designed to restrict phone calls with the area codes: 900, 976, and 540. This device uses a microcontroller to compare the DTMF decoded tones with telephone numbers stored in...

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