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I have a standard NE555P fader circuit powered via a 1.2 V battery boosted by a QX5252 with 220 µH axial inductor (which according to the datasheet should be giving an output of 3.5 DCV @ 15 mA to the NE555P). The net max. output from the 555 circuit is about 2.5 VDC after an NPN boost, which is OK for my red, yellow, and green LEDs but only dimly lights up blue LEDs.

I wish to boost final power output from the NE555P circuit of 2.5 DCV to 3.0 DCV (or thereabouts) at around 10 to 15 mA.

I'm unsure how to approach the increment problem at such low voltages.

Previously, I didn't provide a schematic, but did provide a description; here is a schematic:

Standard 555 fader using QX5252 power supply 3.5v

Power supply:

Typical solar garden light circuit. 1.2V 2100 mAh battery boosted by QX5252 with 220 μH axial inductor between pins 2 and 4.

According to the datasheet this should result in an output of 3.5 VDC @ 15 mA. QX5252 Datasheet QX5252 Datasheet

I've modified the output of pin 4 with a Shockley diode and a small 603 capacitor, the same mod I created on instinct to get RGB functioning in another project, but maybe there is a better way (?). Anyway, the NE555P doesn't trigger without this mod.

NB: Yes, I know the 555 datasheet recommends 5 VDC in. However, other people's research and my own experimentation proves the 555 can work fine down to 1 V (which is all well and good when not playing with LEDs, which may need between 2 to 3 V at respectable mA levels to get desirable brightness).

Fader:

Standard 555 fader circuit. Pin 3 (output) via 50 kΩ pot plus pins 2 and 6 to BC547 base, pins 4 and 8 to BC547 collector. A 100 µF cap between pins 1 and 2. A LED between BC547 emitter and pin 1 (optional mod for red & yellow LED = appropriate resistor between BC547 emitter and + lead of LED). Power + to pin 8, - to pin 1.

NE555P Datasheet

Options:

Playing with pin 5 on the 555 seems to be a possible option to increase the DCV output of pin 3, but I could not find any examples on the internet, just a throw away comment that pin 5 is rarely used in circuits but can be used to modify the 2/3 output of pin 3. Somehow I don't think that is going to solve my problem. Instinct is telling me that a simple mod around the BC547 might be a better option (?).

Any pointers to websites or preferably the provision of schematics to advance my project would be most appreciated.

About me:

I'm just a retired old fart with limited resources who last played with solid state electronics when SCRs had just become available but didn't get the opportunity to play with them (late 1960s?). I have a fair amount of catching up to do in understanding ICs etc. Apart from the basic VRI formulas I'm at a bit of a loss.

My current aim is to make low powered & solar powered toys and novelties for my grandkids (and me!), and share whatever I create on the internet (once I figure out how to edit videos and have something worth sharing).

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    \$\begingroup\$ You may know what a "standard fader circuit" is but I certainly don't. Please provide a schematic, using the built-in schematic editor. Please tell us exactly which part number of the 555 you are using. Please provide links to the manufacturer's datasheets for the 555 and the QX5252. \$\endgroup\$
    – Elliot Alderson
    Commented Jan 30, 2020 at 21:07
  • \$\begingroup\$ Thankyou for your post. Apparently you can't assist me in resolving my issue with power boosting at low voltages. \$\endgroup\$
    – pfc
    Commented Jan 30, 2020 at 21:20
  • \$\begingroup\$ you need a boost DC-DC regulator. They can be had for quite cheap, especially ones with a fixed 5v output, like those used by cheap 18650-based USB power banks. I've even seen one in a AA-USB phone charger at dollartree. \$\endgroup\$
    – dandavis
    Commented Jan 30, 2020 at 21:25
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    \$\begingroup\$ Edit your question and draw us a schematic using the built-in schematic editor. The exact version of the 555 you're using is an important detail to include because the CMOS versions can behave very differently to the bipolar ones in certain situations. \$\endgroup\$
    – brhans
    Commented Jan 30, 2020 at 21:41
  • \$\begingroup\$ If you don't want to use the schematic editor (it is slow and cumbersome) just draw it on paper and take a photo. \$\endgroup\$
    – bobflux
    Commented Jan 30, 2020 at 22:26

2 Answers 2

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Why are you using an old fashioned NE555 that needs a 5V supply, its high current drains the battery and its output high voltage is not high enough, when a Cmos 555 (LMC555 or TLC555) works perfectly from down to 2V, uses almost no battery current and its output is rail-to-rail?

Most of my solar garden lights use the same QX5252 IC as yours but use a 82uH inductor that has low resistance for good brightness. I replaced the low capacity Chinese battery with a modern Energizer or Duracell Ni-MH type for it to light an LED all night long, if the solar panel has a glass top (plastic ones get sunburned) and is at least 1.5" square.

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  • \$\begingroup\$ Last time I played with solid state electronics (over 50 years ago) ICs didn't exist! Last year I decided to play around, got on the internet and all the circuits specified a NE555 for fader circuits, though using a 9 volt battery. I've only using the NE555 to control fading of the LED.The NE555 will run as low as 1v (thats proved)., but its pin 3 will be 2/3 of input as far as I can figure out. Apparently pin 5 can be used to fiddle with the output of Pin 3. \$\endgroup\$
    – pfc
    Commented Jan 31, 2020 at 17:02
  • \$\begingroup\$ The NE555 is an antique part that uses a high power supply current all the time, a minimum supply voltage of 4.5V and a logic high voltage that is 1.3V less than its supply voltage even when it has no load. It was first sold 48 years ago. You should use a modern Cmos 555 that has the benefits I mentioned above. \$\endgroup\$
    – Audioguru
    Commented Feb 1, 2020 at 18:32
  • \$\begingroup\$ Cheapest CMOS555 found on ebay was AU$10 plus AU$7 postage. TS555 about AU#1. NE555, 10x for AU$1. The 555 chip is not my issue. Its output is boosted by the BC547, which I want to increase by 0.5 of a volt (1/5th), I know that can be done, but not sure what is the best way (theory) to go about it (eg: add another NPN or PNP?). \$\endgroup\$
    – pfc
    Commented Feb 3, 2020 at 15:48
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    \$\begingroup\$ ebay sells fakes and rejects. Today at Digikey, one genuine TLC555 in a surface-mount package costs $0.75US and in a through-holes package is $0.84US. \$\endgroup\$
    – Audioguru
    Commented Feb 4, 2020 at 17:00
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It may be overkill for your project, but I would try with a DC/DC boost convertor like an LT1307, you can find a recommended circuit on fig. 1 of the datasheet:

https://www.analog.com/en/products/lt1307.html

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  • \$\begingroup\$ I did a parallel test with a 5v booster and the QX5252 using RGBs and fully charged brand new 1.2v batteries (2100 mah). Only got 8 hours from the 5v booster, got in excess of 135 hours from the QX5252. I've been searching for an IC that has a similiar efficiency but outputs 5v instead of 3.5V. I'll investigate your suggestion. Thanks. \$\endgroup\$
    – pfc
    Commented Jan 31, 2020 at 1:09

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