Converting 20V signals to 3.3V - Page 1
Forum section: electronics / beginners

author: rea5245
author_rank: Frequent Contributor
timestamp: January 04, 2013, 02:46:06 pm
content: Hi,I want to hack my old Heathkit clock by adding a WWVB (US radio time) receiver. I need to sense which segments are currently illuminated on the display to figure out what time the clock is set to. The signals to the VFD are ~20V, and the microcontroller can tolerate 3.3V.I'm aware of some level conversion techniques (voltage divider or a diode protecting the uC with a pull-up resistor to 3.3V). But since there are 3 1/2 digits, there are a lot of segments whose levels I need to shift. I'm hoping to find an IC that can shift several of them. Is there such a thing?Thanks,Bob

author: PA0PBZ
author_rank: Super Contributor
timestamp: January 04, 2013, 03:27:28 pm
content: The display is probably multiplexed (which model is it?), so you will not find a static signal on the display to find out what time it is showing. You would need some heavy decoding logic or a microprocessor to do the decoding. Once you are thinking of using a microprocessor it would me much easier to replace what's in there with a pic and go from there.

author: rea5245
author_rank: Frequent Contributor
timestamp: January 04, 2013, 03:40:31 pm
content: Hi PA0PBZ,Since this is a Heathkit, I have the schematic. The display is definitely not multiplexed. The controller chip has seven pins for the ones digit, seven pins for the tens digit, etc.I thought about replacing the insides entirely, but rejected the idea quickly. I can make a digital clock from scratch just as easily (more easily, actually). The fun here is in keeping the Heathkit hardware intact while nondestructively augmenting it.Anything from Heathkit is vintage electronics. I have no wish to destroy it.- Bob

author: SeanB
author_rank: Super Contributor
timestamp: January 04, 2013, 03:46:17 pm
content: You do not need all 7 segment drives, just enough to decode the number uniquely. For the 10's of hours you need 2 digits for example, and for the hours you need to decode at most 6 lines. Can you post the schematic, if it uses data drivers then there will be TTL drives to them.

author: rea5245
author_rank: Frequent Contributor
timestamp: January 04, 2013, 04:06:28 pm
content: Hi Sean,Yes, in fact the clock chip (a National Semiconductor MM5316) has only 6 pins for the 10s of minutes and one pin for the 10s of hours (plus a pin for AM). I didn't want to get into those nitty gritty details, because they're really a distraction from the main issue, which is that there are lots of pins to convert from 20V (actually 22V) to 3.3V.There's no TTL anywhere in this clock. There's only the single clock chip that operates at 22.3V. You can see the schematic athttp://www.k7jrl.com/pub/manuals/hk/www.tech-systems-labs.com/schematic/GC-1107SCH.jpg.- Bob

author: SeanB
author_rank: Super Contributor
timestamp: January 04, 2013, 04:43:37 pm
content: Use some CD4050 chips to do the level translation, using a resistor network on the inputs to scale, or just drive them direct via a 100k resistor. I have found that most old CMOS will work happily at 20V, although they may run warm. The 100k resistors will limit current in that case. 6 buffers per package, and it is available in DIL or SO packages. 5V output can drive a micro directly, you may want to get a few shift registers and run the bits in to save micro pins. At the data rate from the clock you can do the input quite slowly and just read 3 times and if all are equal then no change in time.

author: rea5245
author_rank: Frequent Contributor
timestamp: January 04, 2013, 10:52:53 pm
content: Thanks Sean.

author: SeanB
author_rank: Super Contributor
timestamp: January 05, 2013, 05:15:41 am
content: I looked at my clock and it uses a similar chip. Seems variants are quite common still, I see them in cheap clock radios all the time, though they do run hot driving a LED with a 15V supply.