counting input from a massive array of simple sensors

[Avital]

Hi,

I need help figuring out a relatively basic circuit, I'll break my
question into two parts:

1) Simple 8-sensor case

I have a circuit that takes a clock signal from a 555 timer and feeds
it into a 7490 counter which generates a 3 bit BCD number (I ignore
the 4th bit) that's fed into a 74151 (8:1 Multiplexer). My sensors
(all very simple and independent), generate TTL level output of logic
'1' and '0', these inputs are fed into the multiplexer inputs. As the
count comes into the multiplexer, it "connects" each sensor input to
the output in turn.

**I would like to be able to count these inputs and display that
count**

I have a set of 7-segment LED displays that are driven by chained
7490s (and the 7-segment decoders obviously - 7448s), all that is
required to set the number is a serial set of pulses into the clock
input of the first 7490 in the chain. This is NOT a rigid requirement
of my circuit - just the way things are set up right now.

My thinking behind the setup was to simply feed the clock input of the
"master" 7490 that drives the LED displays from the output of the
multiplexer. This caused erratic counts, so I added a "debounce"
circuit in between the multiplexer output and the "master" 7490
clock-input. This seemed to give less erratic counts (this is, after
all on a breadboard with relatively loose wiring...noise is to be
expected, or so I figured.) I also hooked up the chained 7490s feeding
the LED displays to reset them to the '0' digit whenever the count
reached 8 on the multiplexer by using an AND gate and the 'C' input on
the 7490 that feeds it.

This approach is not working, I'm not sure why and I'm looking for
a) suggestions on current design problem
b) better ways of doing what I want it without making major changes to
the entire circuit

2) Extension of (1) to 128 or more sensors

Say the scheme figured out for part (1) works, will it extend to 128
or more sensors? What is the best way of doing this? I realise that a
circuit of this type is better designed with a PIC of some sort but at
this stage, I would like to implement it with "off-the-shelf" TTL and
passive components.

Thank you for any help in advance!

**PLEASE USE THIS EMAIL ADDRESS INSTEAD:**

[email protected]
(remove the letter that sounds like "wye" and the words "nospam" to
get actual email)

Avital

 

[R. Steve Walz]

Hi,

I need help figuring out a relatively basic circuit, I'll break my
question into two parts:

1) Simple 8-sensor case

I have a circuit that takes a clock signal from a 555 timer and feeds
it into a 7490 counter which generates a 3 bit BCD number (I ignore
the 4th bit) that's fed into a 74151 (8:1 Multiplexer). My sensors
(all very simple and independent), generate TTL level output of logic
'1' and '0', these inputs are fed into the multiplexer inputs. As the
count comes into the multiplexer, it "connects" each sensor input to
the output in turn.

**I would like to be able to count these inputs and display that
count**

I have a set of 7-segment LED displays that are driven by chained
7490s (and the 7-segment decoders obviously - 7448s), all that is
required to set the number is a serial set of pulses into the clock
input of the first 7490 in the chain. This is NOT a rigid requirement
of my circuit - just the way things are set up right now.

My thinking behind the setup was to simply feed the clock input of the
"master" 7490 that drives the LED displays from the output of the
multiplexer. This caused erratic counts, so I added a "debounce"
circuit in between the multiplexer output and the "master" 7490
clock-input. This seemed to give less erratic counts (this is, after
all on a breadboard with relatively loose wiring...noise is to be
expected, or so I figured.) I also hooked up the chained 7490s feeding
the LED displays to reset them to the '0' digit whenever the count
reached 8 on the multiplexer by using an AND gate and the 'C' input on
the 7490 that feeds it.

This approach is not working, I'm not sure why and I'm looking for
a) suggestions on current design problem
b) better ways of doing what I want it without making major changes to
the entire circuit

2) Extension of (1) to 128 or more sensors

Say the scheme figured out for part (1) works, will it extend to 128
or more sensors? What is the best way of doing this? I realise that a
circuit of this type is better designed with a PIC of some sort but at
this stage, I would like to implement it with "off-the-shelf" TTL and
passive components.

Thank you for any help in advance!

**PLEASE USE THIS EMAIL ADDRESS INSTEAD:**

[email protected]
(remove the letter that sounds like "wye" and the words "nospam" to
get actual email)

Avital

-------------
You want a faster clock so you can use higher order counter bits as
address and use lower bits for preventing timing conflicts and for
gating. Then you can speed it up for more sensors without fear.

-Steve

 

[Avital]

-------------
You want a faster clock so you can use higher order counter bits as
address and use lower bits for preventing timing conflicts and for
gating. Then you can speed it up for more sensors without fear.

-Steve

Could you be more specific? I realise I can use a larger count to work
with more sensors. Was wondering if there are better ways of doing
this? And, ideas on why the current design does NOT work.

Avital