Incremental (digitally controlled) Timer

[Marcus]

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). By the CLOCK switch I can decrement the binary value outputted by the 4516.
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 attached schema, there are 4 separated block. The first determine 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 value (in binary) of the 4516. So if t is the time when the 4516 output binary value is 1, x is the binary value outputted by the 4516 and tf is the desired pulse time, then tf = t/x (more hight is the binary value, less is the time pulse).
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 replace the realys? I'd lookup for CD4066 (quadruple bilateral switches in 14 pin DIP package) or MAX323 (two analog bilateral switches normally open 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.

Thank you in advance

[kam]

i doubt that you will find some component with 0 resistance. Nevertheless, mosfets have on resistance in the range of mOhms which is very good i think. So yuo can replace the transistor-relay set with a low-side switch mosfet... Hopefully this will work. Actually i'm quite sure that it will work (if i understand correctly what you wish to do).

[Marcus]

No component has zero resistance, neither the realys :-). The minum resistence obtainable from the combinations is the "total parallel", that is 100.000/15 = 6666.67 ohm, so a mosfet resistence of 50 ohm (or less) is negligible.
I'm not so pratice with mosfet: I assume that them must be properly polarized to work as switch in the low Source-Drain resistence zone.
Have you some suggestion about the mosfet to use and the relative polarization?

Thank you in adavance

[kam]

The first i encountered



google

[Marcus]

Thank you kam! Let me do some simulation using 4 MOSFET gated asynchronously to se the pulse time result.
The circuit work at 12VDC: I think that a MOSFEF with 16 <= Vds <= 30 can be suitable, don't so?

[Marcus]

Find attacched the schema and the LTSpice IV simulation results.
On the top the NE555 trig signal, on the bottom the NE555 out.
I've simulated the 4516 binary count, using four voltage generator where each of them have a frequency that is 1/2 of the precending.
I've change the value of each single resistor, in the "resistor net", to 86K, because the trig frequency is 1 Hz and the pulse length must be just a little short than 1 second.
So far, I'm satisfied. Now I have to buy the four MOSFET that can fit the proper parameters.
Can you suggest me where I can find a comprehensive list of MOSFETs and their related parameters, including the package? I would like to go to the component shop with a clear idea about what I have to buy.

[kam]

i usually shop from futurlec, but the service is not very good there. they may delay to send the package, so if you shop, make sure that you constantly send them emails asking them if the items are in stock, and when they will send it. because you may order 10 items, and 1 is not in stock, and you get no response (i know it is very bad). but they have good list of items and good prices (especially shipping).

2nd choice is farnell... very good shop, totally reliable, kinda expensive in transportation costs. better ask them what is your local dealer.

[Marcus]

I've solved replacing the old CD4066 (used in the first test) with another. The first CD4066 was (and currently is) bad.