Generating Clean Pulse.

[Lord Loh.]

I have made a keyboard using diodes. The output is in binary. The keyboard is hexadecimal and has 5 outputs. The 5th output is a "key press line". It is essential to differentiate between a 0 and "no key pressed"

My next plan is to put two 4bit registers as a buffer to store an 8bit word.

The 4 register shall be selected by the output of a T filpflop to store the MSB in one register an the LSB in another.

Now since my micro swicthes are noisy, when I attempt to press the key once the key contacts are made more than once.

A debouncer can take care of the time period between the contacts, but not for repeated accidental contacts.

So I am thinking of putting a monostable multivibrator that shall activate on the first key stroke and disable all inputs for ~200ms.

I have two options:
1. 555
2. 74121

The 555 is immune to repeated pulses but not to extended pulses. So if the repeated pulse occurs after the fall of the output, it shall retrigger.

I do not know of the 121 but the datasheet tells that it is edge triggered.

Can anyone suggest a way out ?

I need to have a single key stroke. The person is tempted to press the key harder when the first keystroke does not work. I cannot do away with the switches either.

Please help.

 

[IanP]

74121 is also immune to repeated pulses (74122 and 123 are not).
555 is falling edge triggered, 74121 has 2 inputs: A and B and can therefore be trigged at rising or falling edge, at your selection. From that point of view, 74121 seems to be more universal option..
Regards,
IanP

 

[VVV]

This is what debouncing does, it eliminates the extra pulses generated by the contacts opening and closing a few times before firm cotact is made. Generally, 20ms is used.

The best debouncing is a D flip-flop, clocked at 50Hz. The key is connected to the D input and the Q and Q/ outputs will be clean. Try it.

 

[Lord Loh.]

Good idea... Thanks man... I was thinking of only monostable multivibrators....

The text book I read had debouncers with RS Flip Flops without any clocking... Pity I couldn't see beyond that.