A CMOS gate has an output impedance of a few hundred Ohms, so if it sources more than a few mA the output voltage drops significantly. Whether that drop is important would depend on the application. Here, it doesn't matter much. With Q1 used as a saturated switch having a rule-of-thumb current gain of 10, the designed base current would be 3mA for a 30mA collector current, so the gate output would drop only ~1V. Paralleled gates unnecessary in this case for a 30mA collector current. But unused gates must not have their inputs floating, so it's just as easy to parallel the gates as not.Think the buzzer draws 30ma, will I still need to parallel the outputs of nand gates? What sort of current can you draw from single gate?
The CD4011 can source/sink roughly 1 mA per gate. There are logic families that can sink considerably more current. For instance, the 74AC/ACT families can source/sink about 24 mA per gate. It will consume more current while powered, but since it will only be powered when the door is open, who cares? I would recommend the 74AC132 as it has Schmitt trigger inputs -- those will work much better in an oscillator (particularly a low speed oscillator) than normal inputs because of the hysteresis they have (should you decide you really want a pulsing buzzer).Thanks Alec_t, much appreciated. That's a better circuit, understand now that the diodes in circuit are for suppression incase there is any spikes caused by buzzer. Think the buzzer draws 30ma, will I still need to parallel the outputs of nand gates? What sort of current can you draw from single gate?
Have ordered a 230v AC to 5v DC 700ma power supply, i know its a bit high on current output, only one i could get though. Circuit will run of 5v, don't think it will change any of the component values you have included, or am I wrong?
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by Jake Hertz
by Luke James
by Luke James