Soil Moisture Reader/ Ohm Meter (detailed)

audioguru said:

RED CIRCUITS Version II uses an ordinary CD4093 Cmos gate IC and uses a 3V battery. When the battery voltage drops below 3V (it will drop to below 2V) then the CD4093 does not work. If you increase the supply to 5V (but a battery does not produce 5V) then the typical output current of the CD4093 gate will be only 3mA which is very dim. The current will be only 3mA because datasheets from Texas Instruments show it and the ESR calculates to be 1067 ohms, not 150 ohms.

My idea is to replace the CD4093 with an SN74HC132 ...

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I agree but, I was assuming the same as you suggested for 2V which is the 'HC132 which typically below 150 Ω with some vendor variance.


'HC132 from TI at 2V , the ESR (or RdsOn) is 100 Ω typ. but worst case much higher which few would be but not explicitly guaranteed or spec'd as such.
ESR drops to 50Ω with higher Vcc.

I look at the detailed curves of output current of CD4xxx and 74HCxxx logic ICs on datasheets from Texas Instruments. You can see the output current at various supply voltages into a dead short or into an LED where you can select its forward voltage.

Since your hearing's and your visions's sensitivities are logarithmic then doubling or halving the loudness or the brightness is only a small change.

the 74HCxxx series, from TI shows 50 Ohms at 5V and 100 Ohms typ at 2V. ESR not 1067 ohms.

But one should expect wide tolerances, unless one is on a tight budget for current.
A RED HB LED at 2V ( ~15Ω ESR) and the HC132 ( ~150Ω ESR)

However the coin cell if used will have an ESR of several kΩ , which dominates the current limit.


But if a primary large Lithium cell of 3V were used, one could expect
(3-2)/ 165Ω nom = 6 mA with a 3V cell. The LED would be very dim at 2V as it's actual threshold is 1.85V.

Sorry, I dont understand your contradiction on HC132 ESR.

The ESR of the CD4093 used in the Red Circuits project is 1067 ohms when the battery is 3.0V. I made one to confirm the LED is very dim and the project stops working when the battery voltage drops a little. I made many with my improved circuit using SN74HC132 ICs and they work much better.

I made some LED chasers with an SN74HC4017 driving the anodes of 2V red LEDs and a transistor blinking the cathodes for a short duration. When the two AA alkaline cells battery is new it produces 3.2v. Subtracting the 2V LED and 0.1V transistor saturation the resulting 1.1V is shared between the output resistance of the IC and a 22 ohm current limiting resistor. The current is 24mA which is a little less than the maximum allowed output current from a 74HC IC. Then the ESR of the IC is 23.8 ohms.
As the battery voltage runs down of course the LEDs become a little dimmer and I replace the cells when each is about only 0.75V so the LEDs are still lighting when their forward voltage is maybe only 1.4V. The oscillators made from a 74HC04 and the 74HC4017 still work when their supply is only 1.5V.

Thanks audioguru for your details. I am very glad your modifications worked perfectly.

My only point was the transistor as a switch was redundant.
From your measurements and calculations ( as I read) your transistor drops 0.1V and the SN74HC132 drops 0.57V (= 23.8 Ω *24mA)

Essentially I am saying this CMOS driver could replace R7 = 15Ω and the effect Vce(sat)+ ESR of the transistors.

Right? But no need to change your design. Just a minor point. ( of course AND function with 2Hz Osc would be gated. )

My circuit needs the two transistors as an AND gate so R7 is also needed.
My circuit dims the LED to nothing when the soil is damp and the blinking LED becomes brighter as the soil dries. The RED circuits do not dim the LED, it is on or off. The first RED circuit did not blink the LED so the battery died soon when the soil became dry. The second RED circuit copied my blinker oscillator.

I was thinking of similar except LED off when dry and flashing slow when almost dry and faster when moist.

I had fun making a chaser one weekend for a big band in Winnipeg that spelled the band's name one letter at a time with large lamps. ( circa' 75 )

To make it interesting, I had a few switches for the drummer so he could control the beat by slider or switch to drum mike and have it clocked by the kick drum.

They got tired of changing Triacs when the rodie switched from 100W to 500W floodlights and they sequenced it a little too fast. ( but never told me)