Electronic Design

Bob's Mailbox

Dear Bob: I was just looking through Electronic Design and noticed something in "Bob's Mailbox"* about the 555. Interestingly enough, I did an improved version of the then NEC555, or ICM555. The TI TLC555 all-CMOS timer was designed using a "brand new" TI CMOS process, intended to fit into the same socket as the NEC555 or ICM555, but it offered improved performance. (Yeah, but not at an 18-V supply! What voltage did your design work down to? Let me see—the NSC LMC555 will run from 15 V down to 1.5 V. It's alleged to run on 5 V, even up to 3 MHz, while running on less than 1 mW. Not too poor, although I have never run one myself. Did we steal the design from you, or whatever? When? I have no idea. I do not know who "designed" our version. Hans Camenzind claims he cooked up a low-voltage version of the 555. I never checked to see if it worked down at 1.5 V. I have no idea which version—yours or ours or Hans'—will work best in difficult applications. If yours is still selling, it must be pretty good. /rap)

I know this device well. The TLC555 version of the 555 device line is my design. I designed it in 1983 while I was employed by Texas Instruments. It is great to know that this product has stood the test of time. In fact, you can still pick up one at your local Radio Shack today.

John Morgan (via e-mail)

Pease: Yeah, but can you buy them in Akihabara? I have bought my good old LM337 in a shop in that Tokyo district! And I designed that 27 years ago.

Dear Bob: I have just read your Pease Porridge about theme parks (electronic design, June 7, p. 18). I agree with you completely. But, you forgot in your list one of the most amazing theme parks in the world: Spain. I recommend you visit us. I am sure you will agree with me.

José Vicente Gigante Ripoll (via e-mail)

Pease: Hey, hey, José. I agree that Spain is a wonderful place. I have spent a couple of excellent weekends in Spain. One of my friends and I went up to Segovia, drank a lot of wine, and took a couple of two-hour walks up in the high hills. It was glorious and beautiful! We chose to drive on roads that were supposed to be horrible, but they were quite nice. Then, four of us drove down to Toledo, and that was very pleasant. And, we hiked all around Madrid. We took subways, walked, shopped, and went around Juan Carlos' Park with the great sculpture. I apologize for not mentioning all the possible theme parks. And Spain is an excellent one! I enjoyed the heck out of it with my friends. It was great!

Dear Bob: Hope all is well with you! I ran into an old circuit that scales an analog input and does some gain amplification. Right on the front of the amplifier, it has a protection scheme using two JFETs. They are connected with drain and source shorted, and one protects to the +7.5-V rail and the other to the −7.5-V rail. I understand that the transistors look like protection diodes. But how do JFETs compare to bipolars (I have seen you do this, I seem to recall), compared to just silicon diodes? Is it an issue of voltage drop? Capacitance? Leakage? Is one better than the other? The signals are not fast. It is an old Leeds and Northrup chart recorder.

Chris Eddy (via e-mail)

Pease: Most FETs have to be made with clean processing, so they are pretty low leakage. This was figured out 40 years ago. They are certainly lower leakage than almost any glass diodes—AND the light does not get in. These days, most bipolars are made with good, clean processing. If you get some 2N3904s, 2N4250s, or 2N3707s, their C-B junctions (leave the emitters open circuit) are just as good. The C-B junction is not blindingly fast, to turn ON or OFF, but for this job, that's not a problem.

Comments invited!

[email protected] —or:
Mail Stop D2597A, National Semiconductor
P.O. Box 58090, Santa Clara, CA 95052-8090

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