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setq's comments

setq | 8 years ago | on: Show HN: GreenPiThumb – A Raspberry Pi Gardening Bot

This is a really cool idea actually. However you're likely talking sub nanosecond rise times which makes things a little difficult in the signal processing space. For example my scope (knackered old 1971 Tek 475) can barely manage 1.8ns on a properly terminated transmission line which this will likely not be.

setq | 8 years ago | on: Facebook Hits 2 Billion Users

I get shamed occasionally for never having a Facebook account. It's amazing how mindshare drives market share really and gets people to react like this.

Genuinely I couldn't work out how it improved my life in any way. Everyone I know who uses it shows signs of addiction. It's like a cigarette. Perhaps that's it!

setq | 8 years ago | on: Show HN: GreenPiThumb – A Raspberry Pi Gardening Bot

I don't think many people read the data sheets to be honest even some of the professional engineers. It's mainly gluing canned circuits together and see if they work or not. This is a little difficult if you didn't start with analogue and work up.

setq | 8 years ago | on: Show HN: GreenPiThumb – A Raspberry Pi Gardening Bot

Take a look at the output switching on different variants of 555s (there are about 6). This isn't always the case. Not only that if you pull lots of current through them, and 100mA is a lot, then the Pd of the sink transistor is pushed a bit hard. I've let the smoke out of a few over the years working on that assumption.

Good tip: measure voltage between the output pin and gnd or Vcc in both states and then have a think.

Plus when you're not an idiot like I was, a MOSFET is cheaper.

setq | 8 years ago | on: Show HN: GreenPiThumb – A Raspberry Pi Gardening Bot

Yeah the Tektronix HV probes used to come with an aerosol can and you had to fill them up.

Technically frequency compensation is required across all voltage dividers for scopes so not to accidentally create a low pass filter with the parasitic capacitance in the cables and input circuits. It's all quite fun.

Disclaimer: was an obsessive compulsive scope collector for a while.

setq | 8 years ago | on: Show HN: GreenPiThumb – A Raspberry Pi Gardening Bot

Relay has a considerably lower drive impedance than a MOSFET for switching so you end up having to drive the relay with something anyway because the 555 will probably die. Might as well skip the middle man and use a MOSFET which has a crazy high source impedance.

(you need to stick a transistor between the 555 and the relay anyway so you might as well just use a MOSFET)

setq | 8 years ago | on: Show HN: GreenPiThumb – A Raspberry Pi Gardening Bot

Incidentally you can demonstrate this by wiring up the relay so it oscillates i.e. use the NC contacts in series with the coil. Then stick your fingers across the coil. It'll give you a nice shock. Not enough to hurt you but enough to go "hmm, I'm not going to do that again" like licking a 9v battery.

Attempts to measure these spikes on a cheap scope years ago ended up blowing the scope input FET up. Whoops. Good job it was a university owned scope :D

setq | 8 years ago | on: Show HN: GreenPiThumb – A Raspberry Pi Gardening Bot

I love these projects. They are always destined to fail from experience but are great fun anyway. I've tried a few times to do similar things. I'll catalog my disasters quickly:

1. Seedlings need blowing around a bit when they pop out or they get all spindly. Cue wiring up two 80mm PC case fans to a chain of a single astable then two monostable 555's to generate an oscillating wind field that goes on for 5 seconds each direction after a delay of 10 minutes. Dried the compost out, blew most of it it away and then killed the plants dead. No tomatoes for me!

2. Watering robot version 1. Similar to above but with a 74hc390 dividing down the clock so it only ran once every day. Used an unprotected MOSFET to control a small water pump from ebay. Back EMF blew the MOSFET up and jammed it as a short. Emptied the entire water reservoir into the pot, down the wall and into the carpet.

3. Watering robot version 2. Same as above with problems fixed. Apart from I ran out of bipolar 555's so I used CMOS ones which are a little more tetchy about noise. Cue last 555 getting jammed in an on state and the same thing happening. This time, the tupperware box with the electronics ended up getting wet and the wall wart exploded.

Edit: meant to say to the OP - nice work. This is the spirit of all things interesting :)

setq | 8 years ago | on: A Windows Defender bug was so gaping its PoC exploit had to be encrypted

Windows is a rat's nest from hell when it comes to privilege separation. That was probably the only privilege level that allows it to communicate with what it needs to.

Even worse, despite this patch, it's still sitting here running as local system on my box. Total fucking nightmare.

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