Alarm Dialer Circuit

Posted on Aug 23, 2012

This circuit dials a stored DTMF tone sequence from EPROM when a control line is taken to 0 V. ICl is a Schmitt trigger oscillator, running at around 2 Hz. It clocks a 4024 binary counter. The counter`s outputs connect to the address leads of the EPROM. A 2716 was used here, but the choice of EPROM is by no means critical. Normally, the counter is held reset by a logic 1 on its reset pin (pin 2). When the trigger input is sent low, pin 10 of ICl goes low, pin 3 goes high, and the reset is removed from the counter. It then begins to clock, incrementing the EPROM.

Alarm Dialer Circuit
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When moved from address 000000, the data on bit DO of the EPROM changes to a logic 1 and holds the circuit running. The last address should have data 11111110 to reset the circuit to standby.

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