Some Help With Volt Scale Reversal Needed For An Idiot.

Discussion in 'General Electronics Discussion' started by Incony, Apr 4, 2017.

1. Alec_t

3,144
856
Jul 7, 2015
No.
That should do the trick, but do your calculated resistor values take into account the internal components of the meter? By my reckoning the meter probably includes a 5V regulator and at least one series resistor in order to get the set of voltages given in the table in post #5.

Incony likes this.
2. Incony

25
1
Apr 4, 2017

ty... i apprediate your input and patience Alec_t.

so frustrated because i thought the 3914 would do it alone... but i like the idea of using the 3914 to enable two completely different sources to combine in one... i cant think of a less component and less complicated and cheap method of making a rising positive voltage become a falling negative resistance.. ( sorry for the choice of words its how i see it in my head )

i will either source some negative turn on transistors, like the 2N3906`s or maybe SSR`s the better the isolation, and less components needed and the simpler method is where i want to go.

if i isolate the 3914 outputs there can be no reaction between them and what i want to switch in and out... the resistors..

And no... my calculations dont know exactly the internal arrangement of the temp gauge... i know if that wire is open cct.. the gauge reads 0... and just what it does when its connected to the negative coefficient transducer.. and what happens when i take that input instead .. via a resistor to ground... its how i got the resistance values needed,,, i disconnected the single wire to the transducer... since its other side is ground... and connected a resistor instead to ground... and got the gauge to read the scale i wanted for the value of the resistor used.. so that is all i want.. a particular value of resistance that the meter uses.. for a known voltage input to to the 3914.... i dont need really to scale it to 2,5V max... just know that when it reaches 10 ohms or tenth position thats all i have... ie... over 15 PSI a pressure cap starts to vent... so.. it is so unlikely the voltage will ever reach more than that... but i need to know if it is beyond. and stays there... since it will tell me the pressure cap isnt venting... bad news...

likewise, if it stays low for an as yet undefined time.. like 5 minutes for example... i aint getting pressure... that too is bad news .. the coolant will boil... there isnt enough pressure to stop it.

the ideal point and i have an omega 0 to 30 psi pressure gauge semi permanently plumbed in , to know that, 7 psi is in the normal "window" below 5 psi is abnormal.. above 10 is high... those pressures are affected by the pump/// which makes the pressure change fast... until its at idle... when the engine idles... then the pressure is rising or stabilising as the coolant get hot, since i am looking at the effect of of pressure and temp, together.. it reaches a happy medium.. because the pressure cap cannot accept a vacuum.. so it leaks always... a tiny amount.. it takes over two hours from switch off for the pressure to fall to less than 2 psi... because the engine stays hot, and the pressure leak is tiny... but over 15 psi it vents... that itself isnt a good thing... because the drop in pressure can cause the coolant to start to boil... ie.. if the temp reaches 101 degrees C and the pressure drops... the coolant boils. it cant loose temp fast enough not to.

Last edited: Apr 19, 2017
3. Alec_t

3,144
856
Jul 7, 2015
I wouldn't dream of using SSRs. Far too expensive and bulky. 2N3906 (or almost any other low current PNP transistor) will be fine, since the maximum gauge current is only ~20mA.

Incony likes this.
4. Incony

25
1
Apr 4, 2017
TY Alec_T... i will respond when i venture that path.. but it looks like i want to interface with transistors.... my concern would be the turn on and off time... it must be like a switch... not half on or off

5. Alec_t

3,144
856
Jul 7, 2015
The LM3914 comparators do act like a switch. No turn on/off times are specified in the datasheet, but they would be negligibly small. But why are they a concern? There would be no significant heating of the transistors. There is, however, a deliberate slight overlap between the comparator switching voltage thresholds. This is what the datasheet says :-
"When in the dot mode, there is a small amount of overlap or 'fade' (about 1 mV) between segments. This assures that at no time will all LEDs be OFF, and thus any ambiguous display is avoided."

6. Incony

25
1
Apr 4, 2017
ty... i still think its a good, cheap minimum component solution... and really thank Alec_t for the effort in contribution. i will post what eventually consider the best solution to be... but this mo.. i go 3914 with 2N3906`s..

2
0
Feb 15, 2019

2
0
Feb 15, 2019
Is thay really work?

9. Bhalttar

1
0
Feb 27, 2019
HAVE Circuit PCB?