# Need advice on converting NO switch to NC on alarm clock

I'm trying to modify a Lego alarm clock for my kid. It has a select switch to turn the alarm on/off. It also has a NO push button (smack his head) which lights up the display for a short time and also activates "snooze".

My goal is to make the push button briefly open and close the "alarm on" circuit, which will reset it for another 24hrs. Similar to cycling the alarm on/of switch (not fun). I expect this will negate the snooze function (that's good). I would like it to keep the LED light function (I think it will still work).

Basically I need a relay, but I am limited on space. I think a transistor or something similar should work for this, but I would like help on what is available and how to use it.

I attached a crude schematic to help explain. I am able to tap into either side of the push button, but can only reasonably tap into the load side of the alarm switch. It is part of a "switch only" circuit board feeding many switches ("set time", "hours", "min", etc..) with 3V+ common and the return wires to the clock IC/display board are where I can splice in.

• I expect this will negate the snooze function ... what makes you think that ? – jsotola Dec 31 '17 at 2:21
• Any transistor can operate as an inverting switch with a couple resistors or the gnd switch and pull-up R can be swapped – Sunnyskyguy EE75 Dec 31 '17 at 2:21
• An npn with a 3 v capable NC relay seems about all you need. – Passerby Dec 31 '17 at 2:29
• jsotola - The snooze should be negated because if I hit snooze button and also cycle power to the "alarm on" switch it resets for another 24hrs and does not go off again in 5 minutes – Diznaster Dec 31 '17 at 2:31
• The answer is going to be opinion based. My 2 cent answer is that it sounds like you looking for a 'kill' switch or function that resets after 24 hours, but your 5 minute time out is confusing. Could you please re-write your question and boil it down to the basic reset time and the no-response time you need. – Sparky256 Dec 31 '17 at 5:31

Since this is a battery operated thing, you do not want it to use much current. You could use a small P-Channel MOSFET transistor with a low Vgs (<1V), like an FDC640P or perhaps an SSM3J328R.

simulate this circuit – Schematic created using CircuitLab

• Thanks Trevor, you got me in the right direction. I wound up trying a PNP with a 10K-Ohm on the base and it worked, but did something weird with changing the clock to 24hr format. Luckily I completed the base circuit with my body by accident and it worked. So I used a 1M-Ohm instead and it does what I need now. There must be some other logic function on that momentary switch circuit. I am assuming with such a large resistor I will not use much battery power. Let me know if my assumption is possibly wrong (obviously there are other variables I haven't quite figured out). Thanks! – Diznaster Jan 1 '18 at 4:07
• @Diznaster it kind of depends what the resistance is that is pulling the line low. A mosfet does not add any load on the other line like a PNP does so was a safer bet for my answer. The base current is also continuous with the PNP though 1M is only 3uA. But I would be concerned that may not be enough to really turn on the PNP. – Trevor_G Jan 1 '18 at 4:10
• I understand, I like your solution better. I will probably try that since I got an extra clock in case I fried it or broke it trying to take apart. Would that be considered a "depletion" type? – Diznaster Jan 3 '18 at 2:08
• @Diznaster no, enhancement mode – Trevor_G Jan 3 '18 at 10:28

Basically I need a relay, but I am limited on space.

I found a reed relay as small as 4x4x9mm. Is that small enough?

Edit: That was the wrong one you needed a SPST-NC: https://www.digikey.co.uk/product-detail/en/standex-meder-electronics/CRR05-1B/374-1307-1-ND/2813400

• Thanks oldfart, I considered that relay but went in another direction. I will make an update with the final solution soon and hopefully a video of how to get this clock apart and do it. Maybe someone else hates the "snooze" and wants to just slap a Lego man's head and get out of bed. – Diznaster Jan 1 '18 at 4:13

you may be able to use an opto-coupler together with a transistor to make a small solid state switch which doesn't require a power supply other than the current you have to pass though the led.

• I can easily get power from the batteries if that helps make it simpler – Diznaster Dec 31 '17 at 3:00