lord loh.
Member
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.
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.
