Clamping 9-volt to 8-volt?


1st step of robotics.
Here is a SpeakJet + TTS256 Text to Speech designed to plug on top…


I must warn you that not all of them are mechanically interchangeable.



Only problem is that this board is entirely powered by the regulator populated on to it. The regulator is “supposed” to be an LDO with ~0.2V dropout.

The Datasheet has an “Arbitrary table” of Min/Max specifications; with this sort of “Chinglish” it is really hard to make any confident statements about this part.

If this board is going to be used for I2C communications, then this extra dropout may need to be accounted for at higher bus speeds and affect the values, and current draw, of the selected pull-up resistors.


Well, that is your interpretation of the Datasheet; and you might very well be more versed at deciphering Chinese Datasheets then I am.

The Datasheet reads that 8V is “arbitrary”; which in English means based on random choice or personal whim, rather than any reason or system. As that is how it reads I can do no better then to provided my own personal “SWAG” on what the device might do without actually testing one.

Do you have any pointers to give to someone, like me, that has a hard time with Chinglish Datasheets?


Here’s the fundamental topology of a circuit that I haven’t seen in commercial equipment since about 1980. I guess you’d call it a “bootstrapped pre-regulator”. I’ve cobbled it together on the bench on several occasions when I needed more power dissipation than a 3-terminal regulator was capable of, but it also allows a much higher input voltage than a 3-terminal regulator can handle on its own.

With R1 = R2 the total voltage being dropped is split roughly equally between the series pass transistor Q1, and the Arduino on-board regulator. The series pass transistor must be able to carry the full load current, and dissipate at least half of the total required power loss. In your application the 2N4400 or 2N3904 may do the job - the BC84x series may be a little low on the current capability.

The resistance values in the base circuit are constrained by the minimum current drawn by the Arduino module. You’ll understand this after you calculate (or simulate) circuit behavior under maximum and minimum battery voltage. To make the resistance values less critical use a high-gain pass transistor, or consider a compound connection (Darlington or Sziklai).



In the sacred texts of the Hebrews and Christians there is a story about the times when everybody spoke the same language. The engineers were having a grand time, making buildings and cities and generally improving life for everybody. Then the managers showed up and determined that everybody should be working to show that they were almost as good as God Himself. God didn’t take kindly to that corporate goal, so He made people start to speak different languages. Engineering communication has been difficult ever since.

Regardless of place of origin, the consensus is that Data Sheets should include a section about “Absolute Maximum Ratings” - the conditions that are likely to cause permanent damage to the component. Data Sheets will also show “Performance Characteristics” - the behavior you should expect from a device, under some closely specified conditions.

Sometimes it is difficult to separate the measured performance values, from the conditions under which the measurements were made . . . .and I think the BL8555 Data Sheet is a prime example of that problem. The word “arbitrary” appears on pg 4 with a reference to “output voltage”. This seems to say that the behavior described in the following table applies equally to all of the output voltage variants of the BL8555.

I believe the first line of the table doesn’t belong in the table. Taken literally, it implies that the BL8555 can make its input voltage range between 1.8V and 8V. Rather, it should be made clear that all subsequent lines of the table describe behavior when the input voltage is between 1.8V and 8V.

(And the “1.8V” value doesn’t make sense. Obviously, if you have the version of the BL8555 that regulates to 5.0V, then most of the entries in the table have no meaning if the input voltage is only 1.8V. Instead of 1.8V, the entry for minimum input voltage should be something like “Vreg + Vdropout”.)

I’ll stick to my earlier statement that this device was intended to operate only up to an input voltage of 8V. With higher input voltages, all bets are off. I can hope that the device doesn’t misbehave as severely as some of the examples I previously listed, but I’ve done enough circuit design to know that each of those examples is a possibility.



Summarizing + adding my own suggestions.

  1. drop with 2 diodes, as suggested already. Use any silicon diodes capable of sustaining current consumption of your adruino (probably around 50 mA)
  2. similar to 1 but using a power LED instead of silicon diodes. Bonus - you get a visual ammeter, that will start blinding you if you short something out.
  3. use external 7805 5V regulator IC, or any equivalent. Since on Arduino the reg is LDO, almost all of the 5 V will get right through.


There are also linear regulators with outputs of 5.2V to 8V that would work well.


Thanks to all that have replied !!
You folks have really helped !!