looking for nano-power timer to replace 7555

Thread Starter

j_farina

Joined Dec 7, 2020
17
Hi,

I have a simple circuit to light a LED when pin 2 of a 7555 timer receives a trigger pulse. The LED stays on for about a minute, then shuts off until another trigger pulse is received. This circuit is battery-powered, and the smaller the battery, the better. The supply current for the 7555 is 60 micro-amps. This is good, but not great. Drawing less current in the quiet state would be better. This is a link to the 7555 datasheet: https://www.renesas.com/eu/en/document/dst/icm7555-icm7556-datasheet

I've been looking at the TPL5110, but the datasheet is difficult for me to understand: https://www.ti.com/lit/ds/symlink/tpl5110.pdf

It says this IC is for "power gating" applications, which (I guess) is different from what I'm doing above (lighting a LED for a minute). It seems to draw only ~35 nano-amps, which would be great. I bought a few breakout boards (Adafruit, Sparkfun, and LowPowerLab) for testing, for example TPL5110 Breakout (lowpowerlab.com) I also have some discrete 5110's. But at this point, I don't know if the 5110 can be used as a replacement for a 7555 in my application, or how. Any guidance would be greatly appreciated.

Thanks,

Joe
 

Thread Starter

j_farina

Joined Dec 7, 2020
17
Maybe I should have mentioned that my existing circuit with the 7555 works fine. The output from the 7555 goes to the base of a transistor which drives the LED.
 

Papabravo

Joined Feb 24, 2006
16,157
Hi, the LED draws 30mA
Now compare 30 mA to 35 nanoamperes. The difference is six orders of magnitude, which is not measurable with less than laboratory quality instruments. The LED, all by itself, will drain the battery before any of the other components are able to make a dent in its capacity.
 

Alec_t

Joined Sep 17, 2013
12,071
As Papabravo says, unless the circuit is triggered very rarely. How frequent are the trigger pulse likely to be?
If you really want to reduce current draw, use a high efficiency LED which should give a perfectly adequate light output at a current of ~1mA.
 

Thread Starter

j_farina

Joined Dec 7, 2020
17
Thanks for the replies. The LED is triggered rarely, maybe once a day or once every few days. The LED is for illumination of a small object. A user activates the circuit by a touch button. The LED on-time cycle is infrequent, maybe no one will activate it for a week, or maybe even a month or more. Or sometimes it will be activated a dozen times in one day. That's why I think a nano-power timer will help extend battery life. Hope this makes sense.
 

crutschow

Joined Mar 14, 2008
27,220
The TPL5110 has a one-shot mode that should work for you.

As an alternate, here's the LTspice simulation of a one-shot that uses a CD4093 Schmitt trigger 2-input NAND gate (quad package).
The CD4093 current is 1μA max.
The one-shot time is determined by the R2C2 time-constant.

1607375797490.png
 
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Thread Starter

j_farina

Joined Dec 7, 2020
17
Wow, thank you crutschow. I just have a tinkerer's knowledge of electronics, so your comment about the one-shot mode of the TPL5110 gives me confidence to keep working on it. Thanks for the LTspice simulation and the schematic. I will also be looking into the CD4093, the 1 micro-amp supply current would be great.
 

Marley

Joined Apr 4, 2016
411
Try using the old CMOS 4538 monostable (or a 4528). Quiescent current should be close to zero. Use the highest timing resistor possible. Greater than 1MΩ and could be upto 22MΩ. Only difference between 4538 and 4528 is that the timing formula for the 4538 is CR which makes calculation easy. Use a non-electrolytic capacitor if possible. Or a tantalum capacitor for low leakage.
 

ronsimpson

Joined Oct 7, 2019
1,230
Analog Devices makes a line of "TimeBoxes" that are low power timers. This one will time 1mS to 4 hours. Power is very low. (60uA)
There are many different devices for different jobs.
1607435014116.png
 

Thread Starter

j_farina

Joined Dec 7, 2020
17
Thank you ronsimpson for the suggestion and diagrams. I had a quick look at the LTC6995 datasheet, and, as you mentioned, the quiescent current would be ~60 micro-amps, which is similar to the 7555. While this is good, I plan to have two other IC's on the board which will be drawing some current in the idle state, and the total quiescent current adds up. So, I'm looking for ways to reduce it whenever possible.
 

AnalogKid

Joined Aug 1, 2013
9,256
What is the power source for the circuit?

What is the LED type? Color? Forward voltage (Vf) rating? Part number?

And, as much as I love the 4093, that TPL part is very nice.

ak
 

Thread Starter

j_farina

Joined Dec 7, 2020
17
Hi AnalogKid. The power source is a 1.5V battery, either AA or AAA (AAA is more compact, if it proves to have adequate mAh for this application). Then a boost converter IC to get the voltage to 5V to power the circuit (MicrOne ME2108A).

The LED is neutral white 4500k with a forward voltage of 3.2 to 3.8V. It's a 5mm LED similar to https://www.ebay.com/itm/100-LED-Le...134667?hash=item25c105380b:g:hm8AAOxybi9Re41j

At the moment, I'm looking for ways to hook up the TPL5110 to function as a one-shot timer. The Adafruit and Sparkfun breakout boards for the 5110 do not appear to support the one-shot mode without modification. The LowPowerLab board linked at the top does support it, and this should arrive in a few days. I would like to see how this functions. Then I plan to solder a discrete 5110 to a board for further testing.
 

LesJones

Joined Jan 8, 2017
3,179
One way to reduce the current during the off time is would be to use the output of the timer to hold a PNP transistor or P channel mosfet switched on. This device could then control the power to the timer. The push button would be in parallel with this device and pulse the power on to the timer. The timer would be designed to start it's timing when power was applied to it. When the timer times out it switches the power to itself off. When in the off state the only current is the leakage current of the transistor or mosfet. I originally got this idea from the Banggood component tester design and have used it in a timer based around an Attiny13a.

Les.
 

crutschow

Joined Mar 14, 2008
27,220
What about the power consumed by the MicrOne ME2108A ?

So following LesJones' suggestion, how about having the push-button turn on the MicrOne ME2108A with a transistor switch, which also triggers the one-shot to keep the switch on and thus maintain power to the converter until it times out?
That will give no circuit quiescent power except for the small leakage current of the switch.
 
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Thread Starter

j_farina

Joined Dec 7, 2020
17
Thanks LesJones and AnalogKid, I appreciate your input. Crutschow, thank you for your help, I don't have the exact current draw for the ME2108 in front of me, but it's quite low, much lower than a 7555. I have tested it, and seem to recall it being somewhere in the 20 micro-amp range, but I'm not sure. I will have to ponder the comments in this thread for a while, due to my limited electronics experience and knowledge.

I've attached a schematic of my basic circuit. This is an early version, I'm working on a somewhat more complicated version with the booster IC ME2108 (5V) so it can run from a single 1.5V battery. I have been running the new test version of my circuit with the ME2108 and 7555 with a single AA battery, and it's still working after 3 months.

This older version simply used 4 AA batteries to get 6V. The round thing on the left is the touch button, when the "dot" in the middle and the surrounding "wire" are touched at the same timesunrisejpeg584.jpgit provides a pulse to pin 2 of the 7555, helped by the transistor T1 (a 3904). The resistor and capacitor at bottom right (R6 and C3) cause the LED to fade-in and fade-out (rather than blink on and off suddenly). This older circuit used a higher-current LED (100mA) rather than the 30mA one in my current design. I selected a Darlington transistor (T3, a 6427) because for some unknown reason it worked better in this circuit than a single 3904. People here will probably spot many mistakes in this circuit, it was built through my own trial and error, with the help of more knowledgeable friends.
 

Thread Starter

j_farina

Joined Dec 7, 2020
17
My new circuit is using a single 1.5 battery, which is boosted to 5V with the ME2108, which makes it possible to drive the LED. (The old circuit and attached schematic used four 1.5V batteries to get 6V, and no booster was needed.) I would rather use one AA plus the booster IC, because it takes up less space than four AA's.

Regarding the use of the Darlington at at T3, when I previously tested a 3904 at this location (T3), the other transistor T2 (another 3904) had the same arrangement of components around it, as shown on the schematic above, with its emitter going to R5, R6, and then ground. I hope I understood your question correctly, maybe I didn't. To put it another way, when I tested the regular 3904 at the T3 location (instead of the 6427 Darlington), the rest of the circuit was the same as in the schematic.
 
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