In article <4h5lu7$eea@nntp.Stanford.EDU>,
   meric@harris.Stanford.EDU (Meric Ozcan) wrote:
>In article <4h3a95$> (Michael 
Covington) writes:
>>Is there a simple, clever digital circuit that does the following?
>>Given N pushbuttons and N digital outputs (probably N=8), when you
>>press a button, the corresponding output goes active and all the
>>other outputs are cleared.  Then it remains in that state until you
>>press another button.
>>I have a solution involving a priority encoder, a monostable, and
>>an octal latch to hold the output, and that's only 3 chips plus 4
>>analog components, but is there something simpler that I'm 
>>I think we can assume that only one button will be pressed at a time.
>>If not, the priority encoder can be included in the circuit.
>>It doesn't absolutely have to be a logic circuit, either... 
>>we're controlling relays.
>>Michael A. Covington                

Assuming each relay has a spare NO contact available and that the power 
supply voltage is  +12V, then the following circuit will be hard to 
beat for simplity. It uses , in addition to any number of relays and 
switches,  2 PNP transistors (2N2907A/2N3906) , 3 resistors (1K/100) 
and 2 rectifiers (1N4002). 

1. All relay coils are connected on one side to 0V 
2. One side of each spare NO contact is connected to the other side of 
the coil. The other side of all spare contacts are connected together 
to the collector of a 2N2907 transistor Q1 with the Q1 emitter 
connected to +V.
3. A 2N3906 transistor, Q2, has the collector connected to the base of 
Q1 and the emitter of Q2 is connected t +V.
4. A 1K resistor is connected from the collector of Q2 to 0V and a 
1K resistor from the base of Q2 to +V
5. One side of each switch is connected to it's corresponding relay 
coil and the other sides are connected together and are connected to 
two series connected rectifiers.
6. the common side of the switches is also connected through a 
100 ohm resistor to the base of Q2. 
  ______________________________________ +12V
 1N4002     |    |        |
 1N4002     1K   e        |    
  |----100--|--b 2N3906   e
  |              c------b 2N2907   
  |              |        c
  |             1K        |__________________
  |              |        |          | 
  |              |        |          |
                 |   |    |     |    |
                 |   =PB1 =NO   =PB2 =NO     
                 |   |    |     |    |
                 |   |____|     |____|
                 |        |          |  
                 |       ( )k1      ( )k2
                 |        |          |
                 |________|__________|______ 0V

Date: 1 Mar 1996 07:21:33 GMT

Original Subject: Re: "Pushbutton" effect, digitally

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