Shack clock contrast adjustment


KI7MWA
 

....And NOW...like Ken, I am answering my own question: having MISread the construction manual, I supposed that "no connection" was one of the intended options when, in fact, THE ONLY TWO OPTIONS specified by the manual are jumper and 220k.  It is right there on page 6..."Install one of the two choices:"


KI7MWA
 

Sorry, wrong numerator...should be maximum 1.56 volts.


KI7MWA
 

If I am reading the schematic correctly, the three options (no connection/jumper/resistor) produce three different results:  with no connection (meaning neither R4 nor a jumper is installed), the contrast control pot is connected to ground, therefore Vcon is connected to ground (through more or less resistance), and the pot does nothing at all; with the jumper, the contrast control pot provides a variable voltage to the Vcon pin between 0 and +5 volts; with a resistor installed, the contrast control pot is one element of a voltage divider, therefore providing Vcon a voltage of between 0 and a maximum determined by the ratio of the maximum resistance of the pot to the sum of that resistance plus the resistance of R4 - in the case of R4= 220k, the maximum would be 5V * (100/(100+220)) = 3.44 volts.

In my experience, with no connection, the LCD demonstrates two states: no contrast (with the contrast control pot fully clockwise I can see characters in the blocks of dots if I look at the LCD from the side, but not if I am looking straight on), and blank (with even a slight shift of the pot, neither characters nor blocks of dots are visible).  


Alan G4ZFQ
 

Okay...  But what is the jumper connecting to such that it performs the same function as installing a 220k resistor??
The Very Fine Manual tells you. You have a choice of how you wire the contrast control.

73 Alan G4ZFQ


Jim Sheldon
 

It does NOT perform the same function - it merely makes the contrast control potentiometer the sole factor in setting the contrast.  The 220K resistor in series with that pot (when the resistor is installed) expands the control range of the pot slightly.  It makes it only a little less critical.  I believe a 330K resistor might even be a better choice than the 220K to widen out the range of the contrast control pot a bit.  It is NOT critical.

Jim, W0EB

------ Original Message ------
From: "entilleser via Groups.Io" <entilleser@...>
Sent: 12/13/2018 9:41:07 AM
Subject: Re: [QRPLabs] Shack clock contrast adjustment

Okay...  But what is the jumper connecting to such that it performs the same function as installing a 220k resistor??


KI7MWA
 

Okay...  But what is the jumper connecting to such that it performs the same function as installing a 220k resistor??


M0RON
 

You fit either the wire jumper or a resistor but not both. 
Andy
--
The universe is made up of Protons, Neutrons, Electrons but contains only one M0RON.


KEN G4APB
 


it is just to accommodate the different spacing between fitting a link header or a resistor. The tracks are linked/

Ken G4APB


KI7MWA
 

As I look at it more closely, in the drawing on page 6, the jumper is not installed in the same place as the resistor would be.  Looking at the board itself, there are four holes in a line, the outer pair to accommodate a resistor, and the inner pair to accommodate the jumper.

I'd appreciate it if someone could 'splain it to me.


KI7MWA
 

Ken, thanks.  You answered your own question at the same time I was answering it.


KI7MWA
 

Hey, Ken!  In the file www.qrp-labs.com/images/clockkit/clock_assembly3.pdf , at the top of page 6, under the heading "LCD Contrast Adjustment" is the following text (along with an illustration of the position of R4 on the Clock circuit board): "Resistor R4 is an optional resistor, which is in series with the top of the contrast adjustment potentiometer R1 and +5V. Typically the contrast voltage required is less than 1V and the adjustment is rather sensitive. If you wish, you can fit a 220K resistor in this position, which will make the contrast potentiometer less sensitive to adjust. Install one of the two choices: a) A 220K resistor at R4 (this resistor is not supplied in the kit, OR b) A jumper wire in the position shown, and NO R4."


KEN G4APB
 

Answered my own question!! That comment was for display brightness. 

The contrast pot R1 is 100k so any value of R4 that reduces its effective travel is helpful in making the pot easier to adjust but the same policy can be applied, i.e starting at 100-Kohms. 180-Kohms should work well. 220-Kohms or 270-ohms may also suit your taste. 


KEN G4APB
 

Where oh where does it say 220K ????????

b) Reduced brightness: You may find the LED backlight of the LCD module too bright for your liking, or if you wish to operate from battery power, perhaps you consider that it consumes too much current. In that case, fit a wire jumper between A1 and A2, and use a suitable valued resistor at the R2 position: experiment, e.g. starting at 100-ohms. 180-ohms has been reported to work well. 220-ohms or 270-ohms may also suit your taste. 


KI7MWA
 

For overly sensitive contrast adjustment the assembly manual suggests either 220k ohms or a jumper at R4.  Strikes me as a pretty wide difference.  Happens I don't have any 220k resistors, are other values functional? What difference (or differences) will different values make?