Project files: Simple power-on delay (with 555 IC)

As mentioned in a couple of previous post I have been looking for a simple delay circuit for headphone amps for a while. The original trigger was the Borbely amp project, but many other circuits benefit from a delay on the output to protect speakers and headphones against turn-on and turn-off transients. My (renewed) search led me to this page which has a great circuit. However, the board also has an onboard headphone jack which I don’t want, so roll out Eagle to do another layout 🙂

I already have made an ATtiny-based delay board that could be used but sometimes you want the bulletproof reliability of a design that doesn’t contain any software 😀 And honestly, using a microcontroller for a simple delay circuit is a bit unnecessary – a standard 555 is just fine.

What is it?
A simple power-on delay circuit that can be used to mute headphone outs, speaker outs or similar to protect against DC transients and also potentially e.g as a B+ delay for tube amps and so on. The board is based on the 555 timer IC in monostable mode.

There are two board versions, one with an onboard relay for headphones and line level signals, and one without a relay for use in other applications and for speakers etc. that require high-power relays. The two boards are identical apart from the size (of course) and the fact that the high-power version has bigger voltage regulator and a bigger protection diode because the relay current may exceed the 100mA that the 78Lxx regulator on the low-power board can supply.

The design has two intentional limitations: One is that the regulator powers the 555 directly, meaning you are in practice limited to using 5V-12V relays. The 555 can handle from 3-18V, but relays are mostly 5V and 12V so normally that’s your choice. However, for most of the intended applications this is just fine and the onboard voltage regulator increases the flexibility a bit (and it can be omitted). The other limitation is that there is only one fixed resistor to set the delay time, so no way to shorten it for testing. However, given the intended application I think that should be OK.

How big are the boards?
The no-relay board measures 1.25″ x 1.75″ (app. 32 x 44.5 mm) and the relay-version is a little longer at 1.25″ x 2.45″ (app. 32 x 62 mm)

What is the status of the boards?
Both boards are v1.0. I haven’t tried the no-relay version yet (prototype boards are in the mail), but the circuits are so close to each other that I am fully confident it will work.

Does it use any special/expensive/hard-to-find parts?
No.

Anything else I need to know?

  • The equation for the delay time is 1.1*Rt*Ct, meaning that a capacitor of 22uF and a resistor of 470k gives a nominal delay of app. 11 seconds (not accounting for component tolerances). If you are unsure about the exact times you need/want, size up the capacitor to the next larger size. Partly because tolerances and leakages in the capacitor may reduce the time and partly because it’s always easier to parallel a second resistor on the back of the board to get a lower value 🙂
  • The header marked “MT” forces the output into mute by simply disconnecting power to the relay. You can skip this feature by simply soldering a bridge across the pads or you can use it for a mute switch. The intention is to have a physical mute switch here, but it can actually also be an electrical switch (transistor) from another circuit. This makes it possible to keep the delay function separated, but still disconnect the output in case of a fault.
  • If you are building the no-relay board an isolated 78xx regulator is recommended to protect against unintentional shorts. If you draw a lot of power (with big speaker relays) or if you use the regulator to drop a lot of voltage, a small piece of aluminium as a heat sink would be required. If you don’t need the regulator because you already have a suitable regulated voltage available, just bridge the input and output pins.
  • If for whatever reason, you need the opposite function of this board, namely that the relay is on during the delay period and then it turns off, then you can simply replace the PNP transistor with an NPN-type with the same pinout (such as a BC54x). Don’t bother asking how I found out that this actually works quite well… 😉

Downloads:
Download design files here (EDIT 11th May 2018: File updated to v1.0a to include a BoM-file as well)

Related information:
Note: Always read the “intro post” for additional important information about my designs.

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Project files: A smaller mains controller…

What is it?
As mentioned a few weeks ago I’ve recently built another control board for switching a mains transformer with a low-voltage (latching) switch. This in a slightly different form factor so that if your application requires it, the board can be stacked with a matching standby-PSU and mains splitter and/or my passive softstart board. It is possible (just) to stack all three boards on top of each other in a 2U/80mm high enclosure or just two boards in a 50mm tall enclosure.

You can decide which standby voltage should be used by choosing the right relay in resistor values and in addition to using a latching switch for engaging the relay, you can also use a DC-voltage between app. 3-30V as the trigger. This input is isolated via an optocoupler and the trigger circuit only requires app. 15mA from the triggering device.

The matching standby PSU board uses the (by now) well-known IRM AC-DC power modules from Mean Well. There are two versions, one for the 3W module which is 100% outline-compatible with the control board and a version for the 5-10W modules where some of the connectors had to be shifted but the mounting holes still fit. The PSU board also provides a splitter-function to give two mains outputs.

How big are the boards?
All the boards are 2” x 2” (app. 51 x 51 mm) – the original theslowdiyer industry standard ™ 😀

What is the status of the boards?
These boards are v1.0 and they all work as expected.

Does it use any special/expensive/hard-to-find parts?
No.

Anything else I need to know?

  • The switch must be a latching type (meaning it stays in either on or off positions) and to turn the relay on you connect the switch so that the + voltage is connected to the switch pin. This turns on a transistor which switches the relay on.
  • The relay is rated for 5A inductive loads, so should be good for transformers up to around 1000VA at 230VAC (to give a bit of safety margin).
  • The optocoupler on the trigger input is fed from a constant-current source (CCS) made from an LM317L voltage regulator. If I was designing a commercial product this would probably be a sacking offence because it’s much more expensive than the alternatives, but for our purposes it works quite well 🙂
  • There are two LEDs on the control board, one to indicate the board is powered and one to indicate the relay is on.
  • The “ext” output is intended for us if you want to feed the unswitched standby voltage to some other circuit. There’s space for a bigger resistor here if you need to drop voltage for e.g. LEDs, but you can also jumper the resistor to just get the raw voltage (or leave the output if you don’t need it).
  • The mains connectors on the standby-PSU are marked as inputs and outputs, but in reality it doesn’t matter what you use as inputs and outputs.

Downloads:
Download design files here

Related information:
Note: Always read the “intro post” for additional important information about my designs.

Remember that these boards use mains voltage. Be careful when mounting and handling them!

Another mains controller…

I’ve designed and built a few control boards for switching on mains (e.g. this and this), because it tends to be a thing that many of my projects need. Good (and good looking!) mains switches are hard to come by, especially for higher currents, so it makes sense to use a lower-voltage switch combined with a relay or an SSR for this duty. An obvious downside to the relay-based approach is that a standby voltage is needed to control the relay, but as described in a previous post there are now several types of switching AC-DC converters able to do that job very cheaply and reliably.

However, more often than not I have found that I prefer to keep the standby PSU separate and so this addition to the control-board portfolio was delberately made smaller and to fit my usual 2”x2” format to make it stackable with my softstart-board. For anything with a large transformer in it, this is a combination that is very useful.

Another addition is an external trigger input (isolated with an optocoupler) which I don’t often use to be honest, but which I could see some potential in anyway. To make this feature a bit more versatile I have opted for the “deluxe-version”, by feeding the optocupler from a constant-current source made from an LM317L. This should mean that it’s not just the usual “12V-trigger” input, but actually it would work with any voltage between app. 3-30V and draw less than 20mA from the triggering device.

“In flight” (or at least on the way) are boards for a matching standby PSU based on the Mean Well IRM power modules – when everything is here and tested I’ll publish some files and more pictures 🙂

Project files: ICEpower integrated amp board

What is it?
The project files for the “all-in-one” (nearly…) PCB for making integrated ICEpower amps shown in the previous post.

How big are the boards?
The board measures 2.65″ x 3.15″ (app. 67 x 80 mm.).

What is the status of the boards?
The board is version 2.1. As mentioned, it’s an old design that I have revised and updated to give it the 2.x version number. I’ve built my prototype on a v2.0 board and made some minor tweaks to that before publishing.
The changes in v2.1. are mostly mechanical (too little space for the input connectors etc.) and then minor touch-ups to the silk screen.

Does it use any special/expensive/hard-to-find parts?
No. The overall circuit is quite simple and only a few parts require a bit of attention.

  • The relays are standard mid-sized “2 form C” contact types. If you’re buying from scratch I’d recommend the Takamisawa RY-12W type, but there are app. 1 million equivalents with similar specs and footprint, so you may be able to get good surplus deals as well :). The coil voltage must be 12V.
  • The voltage regulators are standard 7812/7912 types but as they are mounted very close together I recommend the fully-insulated versions. I prefer the ones made by NJR as opposed to ST because the ST-ones seem to behave a bit strangely sometimes (and yes, I might be imagining this…).
  • See BoM-file for description of other parts and values.

Anything else I need to know?
A few things:

  • The on-board parts draw no current from the negative PSU rail. If you’re not using any external circuitry you can omit the negative rail (regulator etc.). If you build it anyway and get strange results, note that some regulators do not like a “no-load” condition and will give an weird unregulated output if not loaded. You can solder a 1-3k resistor on the bottom if you want for added peace of mind.
  • ASP/ASC-usage: It’s possible to use the board with ICEpower ASC and ASP modules. As these include a regulated +/- 12V AUX supply, you should jumper the regulators and the input resistors. The capacitors and remaining components can be left in.
  • Mute-header: The Mute-header simply brings the two pins required for the module’s mute or standby pins to work to a header at the from of the board to simplify wiring. Refer to the datasheet for the respective modules for details on how to use this, but in general you can switch using a mechanical switch.
  • Heat sinking: There is no heat sinking of the regulators as standard. With a simple preamp and no additional load this should not be necessary, but if you want to draw more power then use a small bit of metal as the heat sink. There is not much space in either direction, so using insulated regulators will once again be an advantage.
  • If you prefer a manual input switch, the board is just about ready and will be presented as part of another project post in a few weeks 🙂

Downloads:
Download design files here

Related information:
Please read the FAQs in the original post as well. The picture below shows my “in progress” prototype amp with the Minipre and a 50ASX-module and gives an idea of the expected layout.

Note: Always read the “intro post” for additional important information about my designs.

ice-int-wip