Incremental Timer

Discussion in 'The Projects Forum' started by ElectroBeginner, Aug 14, 2012.

1. ElectroBeginner Thread Starter New Member

May 1, 2012
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Hi,

The goal of my little project (part of largest project), is to have a output signal (pulse) which durantion depend on the output value (binary) of a digital counter.
The signal is obtained by a NE555 configured as not retriggerable monostable.
The counter is a 4516 binary u/d counter with reset, preset and carry-in options.
By the switch RESET and PRESET I can initialize the 4516, so it start to count from 15 to 0 (the U/D pin is set to GND and the P-reset input are all connected to VDD)
Each output of the CD4516 (Qn) close a NPN transitor to GND so that the related realy can close to NO.
The Realy RLY1 connect the resistor R1 from VDD to the THRS pin of the monostrable
The relay RLY2 connect the resistor R2//R3 from VDD to THRS pin of the monostrable, and so on
As you can see in the schema, there are 4 separated block. The first determne 100 kohm when Q1 is on; the second determine 50 kohm when Q2 is on, the third determine 25 kohm when Q3 is on and the last determine 12.5 kohm when Q4 is on.
When Q1 and Q2 are on then R1//(R2//R3), and therefor R1//R2//R3 so the result resistance is 100/3 = 33.3 kohm.
When Q2 and Q3 are on then (R2//R3)//(R4//R5//R6//R7) and therefor R2//R3//R4//R5//R6//R7 so the result resistance is 100/6 = 16.6 kohm.
When Q1, Q2 and Q3 are on the result resistance is 100/7 = 14.28 kohm, and so on
In other words, the realy connect (or not) the resistance to create a parallel which value is 100 kohm/x where x is the current value (in binary) of the 4516
The final resistance value, applied to the NE555 via the RC2, create a pulse that begins when press the TRIG button and ends when C2 has charger to 2/3 VDD.
I would avoid use relays: the main reason is that they are mechanic and subject to be broken, the second reason is that they are noisly (tic-tac)
Can you suggest me one or more component by which I can substitute the realys? I'd lookup for CD4066 (quadruple bilateral switches in 14 pin DIP package) or MAX323 (two analog bilateral switches in 8 pin DIP package), but the CD4066 has a strange behavior with the resistence and I would avoid to buy couple of MAX323 if they have the same problem of the CD4066.

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2. #12 Expert

Nov 30, 2010
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You have a mistaken connection from pins 2 and 6 to pin 8.
The idea of a weighted analog output based on a digital number is well known.
The way this is wired, the higher the binary number, the shorter the pulse time.
Is that what you intended?

3. ElectroBeginner Thread Starter New Member

May 1, 2012
5
0
Which integratet do you refer? NE555 or 4516?
Oh yes, I'm not an inventor: my only intent is to find a solution for my needs
Yes, you have exactly described the purpose of the circuit. More precisely, if t is the pulse time when binary value is 1, x is the binary value, and tf is the final pulse time, then tf = t/x.

4. tracecom AAC Fanatic!

Apr 16, 2010
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The NE555: pin 8 is Vcc and should not be tied to pins 2 & 6. Pin 4 is also tied to Vcc, which is correct.

5. ElectroBeginner Thread Starter New Member

May 1, 2012
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Thank you for the warning. Find attached the fixed schema.

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6. tracecom AAC Fanatic!

Apr 16, 2010
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Pins 6 and 2 on the NE555 are no longer connected to each other, but they should be.

7. ElectroBeginner Thread Starter New Member

May 1, 2012
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Why pins 2 and 6 should be connected each other?
I think that when the NE555 is in the monostable non retriggerable configuration, the mentioned pins should not be connected together.
But the question is how to replace the relays with other electronic component.

Thank you

8. tracecom AAC Fanatic!

Apr 16, 2010
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I have not studied your schematic in detail. My comment about pins 2 and 6 was made because they were connected together in the first version of the schematic you posted (as they usually are.) Then, when you disconnected them from Vcc, you also disconnected them from each other; I thought that you did that by accident. If it was not by accident, then it is my mistake.

9. ElectroBeginner Thread Starter New Member

May 1, 2012
5
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I've solved replacing the old CD4066 (using in the initial test) with another. The first CD4066 was bad.